diff --git a/clang-tools-extra/clangd/TidyFastChecks.inc b/clang-tools-extra/clangd/TidyFastChecks.inc
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/TidyFastChecks.inc
@@ -0,0 +1,379 @@
+// This file is generated, do not edit it directly!
+// This describes 
+#ifndef FAST
+#define FAST(CHECK, DELTA)
+#endif
+#ifndef SLOW
+#define SLOW(CHECK, DELTA)
+#endif
+
+FAST(abseil-cleanup-ctad, 1.0)
+FAST(abseil-duration-addition, -1.0)
+FAST(abseil-duration-comparison, 0.0)
+FAST(abseil-duration-conversion-cast, 1.0)
+FAST(abseil-duration-division, 1.0)
+FAST(abseil-duration-factory-float, 0.0)
+FAST(abseil-duration-factory-scale, -0.0)
+FAST(abseil-duration-subtraction, -0.0)
+FAST(abseil-duration-unnecessary-conversion, 1.0)
+FAST(abseil-faster-strsplit-delimiter, 0.0)
+FAST(abseil-no-internal-dependencies, 0.0)
+FAST(abseil-no-namespace, 0.0)
+FAST(abseil-redundant-strcat-calls, 1.0)
+FAST(abseil-str-cat-append, 1.0)
+FAST(abseil-string-find-startswith, 0.0)
+FAST(abseil-string-find-str-contains, 1.0)
+FAST(abseil-time-comparison, -0.0)
+FAST(abseil-time-subtraction, 2.0)
+FAST(abseil-upgrade-duration-conversions, 2.0)
+SLOW(altera-id-dependent-backward-branch, 14.0)
+FAST(altera-kernel-name-restriction, 0.0)
+FAST(altera-single-work-item-barrier, 0.0)
+FAST(altera-struct-pack-align, 1.0)
+FAST(altera-unroll-loops, 2.0)
+FAST(android-cloexec-accept, 0.0)
+FAST(android-cloexec-accept4, 0.0)
+FAST(android-cloexec-creat, 1.0)
+FAST(android-cloexec-dup, 0.0)
+FAST(android-cloexec-epoll-create, 2.0)
+FAST(android-cloexec-epoll-create1, -0.0)
+FAST(android-cloexec-fopen, -1.0)
+FAST(android-cloexec-inotify-init, 0.0)
+FAST(android-cloexec-inotify-init1, 0.0)
+FAST(android-cloexec-memfd-create, -0.0)
+FAST(android-cloexec-open, 0.0)
+FAST(android-cloexec-pipe, 1.0)
+FAST(android-cloexec-pipe2, -0.0)
+FAST(android-cloexec-socket, 0.0)
+FAST(android-comparison-in-temp-failure-retry, 0.0)
+FAST(boost-use-to-string, 1.0)
+FAST(bugprone-argument-comment, 2.0)
+FAST(bugprone-assert-side-effect, 1.0)
+FAST(bugprone-assignment-in-if-condition, 0.0)
+FAST(bugprone-bad-signal-to-kill-thread, 1.0)
+FAST(bugprone-bool-pointer-implicit-conversion, 1.0)
+FAST(bugprone-branch-clone, 1.0)
+FAST(bugprone-copy-constructor-init, 0.0)
+FAST(bugprone-dangling-handle, 0.0)
+FAST(bugprone-dynamic-static-initializers, 1.0)
+FAST(bugprone-easily-swappable-parameters, 2.0)
+FAST(bugprone-exception-escape, 1.0)
+FAST(bugprone-fold-init-type, 1.0)
+FAST(bugprone-forward-declaration-namespace, 1.0)
+FAST(bugprone-forwarding-reference-overload, 0.0)
+FAST(bugprone-implicit-widening-of-multiplication-result, 2.0)
+FAST(bugprone-inaccurate-erase, 1.0)
+FAST(bugprone-incorrect-roundings, 2.0)
+FAST(bugprone-infinite-loop, 3.0)
+FAST(bugprone-integer-division, -0.0)
+FAST(bugprone-lambda-function-name, 0.0)
+FAST(bugprone-macro-parentheses, 0.0)
+FAST(bugprone-macro-repeated-side-effects, 1.0)
+FAST(bugprone-misplaced-operator-in-strlen-in-alloc, 1.0)
+FAST(bugprone-misplaced-pointer-arithmetic-in-alloc, 0.0)
+FAST(bugprone-misplaced-widening-cast, -0.0)
+FAST(bugprone-move-forwarding-reference, -0.0)
+FAST(bugprone-multiple-statement-macro, 0.0)
+FAST(bugprone-narrowing-conversions, 1.0)
+FAST(bugprone-no-escape, 0.0)
+FAST(bugprone-not-null-terminated-result, 1.0)
+FAST(bugprone-parent-virtual-call, 0.0)
+FAST(bugprone-posix-return, -0.0)
+FAST(bugprone-redundant-branch-condition, -0.0)
+FAST(bugprone-reserved-identifier, 2.0)
+FAST(bugprone-shared-ptr-array-mismatch, 0.0)
+FAST(bugprone-signal-handler, -1.0)
+FAST(bugprone-signed-char-misuse, 1.0)
+FAST(bugprone-sizeof-container, -0.0)
+FAST(bugprone-sizeof-expression, 1.0)
+FAST(bugprone-spuriously-wake-up-functions, 1.0)
+FAST(bugprone-string-constructor, 0.0)
+FAST(bugprone-string-integer-assignment, 1.0)
+FAST(bugprone-string-literal-with-embedded-nul, 1.0)
+FAST(bugprone-stringview-nullptr, 3.0)
+FAST(bugprone-suspicious-enum-usage, 0.0)
+FAST(bugprone-suspicious-include, 1.0)
+FAST(bugprone-suspicious-memory-comparison, 0.0)
+FAST(bugprone-suspicious-memset-usage, 2.0)
+FAST(bugprone-suspicious-missing-comma, -0.0)
+FAST(bugprone-suspicious-realloc-usage, 1.0)
+FAST(bugprone-suspicious-semicolon, 1.0)
+FAST(bugprone-suspicious-string-compare, 1.0)
+FAST(bugprone-swapped-arguments, 1.0)
+FAST(bugprone-terminating-continue, -0.0)
+FAST(bugprone-throw-keyword-missing, 1.0)
+FAST(bugprone-too-small-loop-variable, 0.0)
+FAST(bugprone-unchecked-optional-access, 1.0)
+FAST(bugprone-undefined-memory-manipulation, 0.0)
+FAST(bugprone-undelegated-constructor, 1.0)
+FAST(bugprone-unhandled-exception-at-new, 0.0)
+FAST(bugprone-unhandled-self-assignment, -1.0)
+FAST(bugprone-unused-raii, 1.0)
+FAST(bugprone-unused-return-value, 2.0)
+FAST(bugprone-use-after-move, 6.0)
+FAST(bugprone-virtual-near-miss, 0.0)
+FAST(cert-con36-c, 1.0)
+FAST(cert-con54-cpp, 2.0)
+FAST(cert-dcl03-c, 2.0)
+FAST(cert-dcl16-c, 2.0)
+FAST(cert-dcl21-cpp, -1.0)
+FAST(cert-dcl37-c, 2.0)
+FAST(cert-dcl50-cpp, -1.0)
+FAST(cert-dcl51-cpp, 2.0)
+FAST(cert-dcl54-cpp, 0.0)
+FAST(cert-dcl58-cpp, 0.0)
+FAST(cert-dcl59-cpp, 0.0)
+FAST(cert-env33-c, 1.0)
+FAST(cert-err09-cpp, -0.0)
+FAST(cert-err33-c, 2.0)
+FAST(cert-err34-c, 0.0)
+FAST(cert-err52-cpp, -0.0)
+FAST(cert-err58-cpp, 0.0)
+FAST(cert-err60-cpp, 1.0)
+FAST(cert-err61-cpp, 1.0)
+FAST(cert-exp42-c, 1.0)
+FAST(cert-fio38-c, 0.0)
+FAST(cert-flp30-c, -0.0)
+FAST(cert-flp37-c, 1.0)
+FAST(cert-mem57-cpp, 0.0)
+FAST(cert-msc30-c, -2.0)
+FAST(cert-msc32-c, 5.0)
+FAST(cert-msc50-cpp, 1.0)
+FAST(cert-msc51-cpp, 2.0)
+FAST(cert-msc54-cpp, -0.0)
+FAST(cert-oop11-cpp, 1.0)
+FAST(cert-oop54-cpp, 0.0)
+FAST(cert-oop57-cpp, 1.0)
+FAST(cert-oop58-cpp, 0.0)
+FAST(cert-pos44-c, 0.0)
+FAST(cert-pos47-c, 1.0)
+FAST(cert-sig30-c, -0.0)
+FAST(cert-str34-c, 0.0)
+FAST(concurrency-mt-unsafe, 3.0)
+FAST(concurrency-thread-canceltype-asynchronous, 2.0)
+FAST(cppcoreguidelines-avoid-c-arrays, 0.0)
+FAST(cppcoreguidelines-avoid-const-or-ref-data-members, 0.0)
+FAST(cppcoreguidelines-avoid-do-while, -0.0)
+FAST(cppcoreguidelines-avoid-goto, -0.0)
+FAST(cppcoreguidelines-avoid-magic-numbers, 5.0)
+FAST(cppcoreguidelines-avoid-non-const-global-variables, -0.0)
+FAST(cppcoreguidelines-c-copy-assignment-signature, 1.0)
+FAST(cppcoreguidelines-explicit-virtual-functions, 0.0)
+FAST(cppcoreguidelines-init-variables, 1.0)
+FAST(cppcoreguidelines-interfaces-global-init, 0.0)
+FAST(cppcoreguidelines-macro-usage, -0.0)
+FAST(cppcoreguidelines-narrowing-conversions, 1.0)
+FAST(cppcoreguidelines-no-malloc, 1.0)
+FAST(cppcoreguidelines-non-private-member-variables-in-classes, -1.0)
+FAST(cppcoreguidelines-owning-memory, 2.0)
+FAST(cppcoreguidelines-prefer-member-initializer, 0.0)
+FAST(cppcoreguidelines-pro-bounds-array-to-pointer-decay, 3.0)
+FAST(cppcoreguidelines-pro-bounds-constant-array-index, 0.0)
+FAST(cppcoreguidelines-pro-bounds-pointer-arithmetic, 1.0)
+FAST(cppcoreguidelines-pro-type-const-cast, -0.0)
+FAST(cppcoreguidelines-pro-type-cstyle-cast, 1.0)
+FAST(cppcoreguidelines-pro-type-member-init, 2.0)
+FAST(cppcoreguidelines-pro-type-reinterpret-cast, -0.0)
+FAST(cppcoreguidelines-pro-type-static-cast-downcast, 1.0)
+FAST(cppcoreguidelines-pro-type-union-access, 1.0)
+FAST(cppcoreguidelines-pro-type-vararg, 0.0)
+FAST(cppcoreguidelines-slicing, -2.0)
+FAST(cppcoreguidelines-special-member-functions, -0.0)
+FAST(cppcoreguidelines-virtual-class-destructor, 1.0)
+FAST(darwin-avoid-spinlock, -0.0)
+FAST(darwin-dispatch-once-nonstatic, 0.0)
+FAST(fuchsia-default-arguments-calls, 1.0)
+FAST(fuchsia-default-arguments-declarations, 0.0)
+FAST(fuchsia-header-anon-namespaces, 0.0)
+FAST(fuchsia-multiple-inheritance, 1.0)
+FAST(fuchsia-overloaded-operator, 1.0)
+FAST(fuchsia-statically-constructed-objects, 1.0)
+FAST(fuchsia-trailing-return, 0.0)
+FAST(fuchsia-virtual-inheritance, 0.0)
+FAST(google-build-explicit-make-pair, 1.0)
+FAST(google-build-namespaces, 0.0)
+FAST(google-build-using-namespace, -1.0)
+FAST(google-default-arguments, 0.0)
+FAST(google-explicit-constructor, 1.0)
+FAST(google-global-names-in-headers, -1.0)
+FAST(google-objc-avoid-nsobject-new, 0.0)
+FAST(google-objc-avoid-throwing-exception, -0.0)
+FAST(google-objc-function-naming, 0.0)
+FAST(google-objc-global-variable-declaration, 1.0)
+FAST(google-readability-avoid-underscore-in-googletest-name, 0.0)
+FAST(google-readability-braces-around-statements, 1.0)
+FAST(google-readability-casting, 1.0)
+FAST(google-readability-function-size, 1.0)
+FAST(google-readability-namespace-comments, 0.0)
+FAST(google-readability-todo, 1.0)
+FAST(google-runtime-int, 0.0)
+FAST(google-runtime-operator, 0.0)
+FAST(google-upgrade-googletest-case, 1.0)
+FAST(hicpp-avoid-c-arrays, 1.0)
+FAST(hicpp-avoid-goto, 1.0)
+FAST(hicpp-braces-around-statements, 0.0)
+FAST(hicpp-deprecated-headers, 0.0)
+FAST(hicpp-exception-baseclass, -0.0)
+FAST(hicpp-explicit-conversions, 1.0)
+FAST(hicpp-function-size, 1.0)
+FAST(hicpp-invalid-access-moved, 5.0)
+FAST(hicpp-member-init, 1.0)
+FAST(hicpp-move-const-arg, 1.0)
+FAST(hicpp-multiway-paths-covered, 0.0)
+FAST(hicpp-named-parameter, 0.0)
+FAST(hicpp-new-delete-operators, 0.0)
+FAST(hicpp-no-array-decay, 4.0)
+FAST(hicpp-no-assembler, -0.0)
+FAST(hicpp-no-malloc, 0.0)
+FAST(hicpp-noexcept-move, 0.0)
+FAST(hicpp-signed-bitwise, 0.0)
+FAST(hicpp-special-member-functions, 1.0)
+FAST(hicpp-static-assert, 1.0)
+FAST(hicpp-undelegated-constructor, 1.0)
+FAST(hicpp-uppercase-literal-suffix, 2.0)
+FAST(hicpp-use-auto, 1.0)
+FAST(hicpp-use-emplace, 1.0)
+FAST(hicpp-use-equals-default, 0.0)
+FAST(hicpp-use-equals-delete, 1.0)
+FAST(hicpp-use-noexcept, 1.0)
+FAST(hicpp-use-nullptr, 2.0)
+FAST(hicpp-use-override, 0.0)
+FAST(hicpp-vararg, 0.0)
+FAST(linuxkernel-must-check-errs, 2.0)
+FAST(llvm-else-after-return, 1.0)
+FAST(llvm-header-guard, 0.0)
+FAST(llvm-include-order, 1.0)
+FAST(llvm-namespace-comment, 0.0)
+FAST(llvm-prefer-isa-or-dyn-cast-in-conditionals, 1.0)
+FAST(llvm-prefer-register-over-unsigned, -1.0)
+FAST(llvm-qualified-auto, 1.0)
+FAST(llvm-twine-local, 0.0)
+FAST(llvmlibc-callee-namespace, 2.0)
+FAST(llvmlibc-implementation-in-namespace, -0.0)
+FAST(llvmlibc-restrict-system-libc-headers, 0.0)
+FAST(misc-confusable-identifiers, 1.0)
+SLOW(misc-const-correctness, 268.0)
+FAST(misc-definitions-in-headers, 1.0)
+FAST(misc-misleading-bidirectional, 0.0)
+FAST(misc-misleading-identifier, 1.0)
+FAST(misc-misplaced-const, 0.0)
+FAST(misc-new-delete-overloads, 1.0)
+FAST(misc-no-recursion, 0.0)
+FAST(misc-non-copyable-objects, -1.0)
+FAST(misc-non-private-member-variables-in-classes, -1.0)
+FAST(misc-redundant-expression, 2.0)
+FAST(misc-static-assert, 2.0)
+FAST(misc-throw-by-value-catch-by-reference, -0.0)
+FAST(misc-unconventional-assign-operator, 0.0)
+FAST(misc-uniqueptr-reset-release, 0.0)
+FAST(misc-unused-alias-decls, 1.0)
+FAST(misc-unused-parameters, 1.0)
+FAST(misc-unused-using-decls, 1.0)
+FAST(modernize-avoid-bind, 1.0)
+FAST(modernize-avoid-c-arrays, 2.0)
+FAST(modernize-concat-nested-namespaces, -1.0)
+FAST(modernize-deprecated-headers, -0.0)
+FAST(modernize-deprecated-ios-base-aliases, 0.0)
+FAST(modernize-loop-convert, 3.0)
+FAST(modernize-macro-to-enum, 0.0)
+FAST(modernize-make-shared, 0.0)
+FAST(modernize-make-unique, 1.0)
+FAST(modernize-pass-by-value, 0.0)
+FAST(modernize-raw-string-literal, 1.0)
+FAST(modernize-redundant-void-arg, 1.0)
+FAST(modernize-replace-auto-ptr, 1.0)
+FAST(modernize-replace-disallow-copy-and-assign-macro, -0.0)
+FAST(modernize-replace-random-shuffle, -0.0)
+FAST(modernize-return-braced-init-list, 1.0)
+FAST(modernize-shrink-to-fit, 0.0)
+FAST(modernize-unary-static-assert, 0.0)
+FAST(modernize-use-auto, 1.0)
+FAST(modernize-use-bool-literals, 1.0)
+FAST(modernize-use-default-member-init, 1.0)
+FAST(modernize-use-emplace, 1.0)
+FAST(modernize-use-equals-default, 0.0)
+FAST(modernize-use-equals-delete, 0.0)
+FAST(modernize-use-nodiscard, 0.0)
+FAST(modernize-use-noexcept, 1.0)
+FAST(modernize-use-nullptr, 2.0)
+FAST(modernize-use-override, 0.0)
+FAST(modernize-use-trailing-return-type, -0.0)
+FAST(modernize-use-transparent-functors, 1.0)
+FAST(modernize-use-uncaught-exceptions, 0.0)
+FAST(modernize-use-using, 1.0)
+FAST(objc-assert-equals, -0.0)
+FAST(objc-avoid-nserror-init, 0.0)
+FAST(objc-dealloc-in-category, 0.0)
+FAST(objc-forbidden-subclassing, -1.0)
+FAST(objc-missing-hash, -0.0)
+FAST(objc-nsdate-formatter, -0.0)
+FAST(objc-nsinvocation-argument-lifetime, -1.0)
+FAST(objc-property-declaration, 0.0)
+FAST(objc-super-self, -0.0)
+FAST(openmp-exception-escape, 1.0)
+FAST(openmp-use-default-none, 1.0)
+FAST(performance-faster-string-find, -0.0)
+FAST(performance-for-range-copy, 1.0)
+FAST(performance-implicit-conversion-in-loop, -1.0)
+FAST(performance-inefficient-algorithm, 1.0)
+FAST(performance-inefficient-string-concatenation, -0.0)
+FAST(performance-inefficient-vector-operation, 0.0)
+FAST(performance-move-const-arg, 2.0)
+FAST(performance-move-constructor-init, 0.0)
+FAST(performance-no-automatic-move, 0.0)
+FAST(performance-no-int-to-ptr, 0.0)
+FAST(performance-noexcept-move-constructor, -1.0)
+FAST(performance-trivially-destructible, 0.0)
+FAST(performance-type-promotion-in-math-fn, 1.0)
+FAST(performance-unnecessary-copy-initialization, 4.0)
+FAST(performance-unnecessary-value-param, 1.0)
+FAST(portability-restrict-system-includes, -0.0)
+FAST(portability-simd-intrinsics, 2.0)
+FAST(portability-std-allocator-const, -0.0)
+FAST(readability-avoid-const-params-in-decls, 1.0)
+FAST(readability-braces-around-statements, 1.0)
+FAST(readability-const-return-type, 0.0)
+FAST(readability-container-contains, -1.0)
+FAST(readability-container-data-pointer, 0.0)
+SLOW(readability-container-size-empty, 17.0)
+FAST(readability-convert-member-functions-to-static, 0.0)
+FAST(readability-delete-null-pointer, 0.0)
+FAST(readability-duplicate-include, 1.0)
+FAST(readability-else-after-return, 1.0)
+FAST(readability-function-cognitive-complexity, 0.0)
+FAST(readability-function-size, 1.0)
+FAST(readability-identifier-length, 1.0)
+FAST(readability-identifier-naming, 3.0)
+FAST(readability-implicit-bool-conversion, 2.0)
+FAST(readability-inconsistent-declaration-parameter-name, -0.0)
+FAST(readability-isolate-declaration, -0.0)
+FAST(readability-magic-numbers, -0.0)
+FAST(readability-make-member-function-const, 2.0)
+FAST(readability-misleading-indentation, 0.0)
+FAST(readability-misplaced-array-index, 0.0)
+FAST(readability-named-parameter, 0.0)
+FAST(readability-non-const-parameter, 1.0)
+FAST(readability-qualified-auto, 1.0)
+FAST(readability-redundant-access-specifiers, -0.0)
+FAST(readability-redundant-control-flow, 1.0)
+FAST(readability-redundant-declaration, 1.0)
+FAST(readability-redundant-function-ptr-dereference, 1.0)
+FAST(readability-redundant-member-init, 0.0)
+FAST(readability-redundant-preprocessor, 1.0)
+FAST(readability-redundant-smartptr-get, 5.0)
+FAST(readability-redundant-string-cstr, 1.0)
+FAST(readability-redundant-string-init, -0.0)
+FAST(readability-simplify-boolean-expr, -1.0)
+FAST(readability-simplify-subscript-expr, -0.0)
+FAST(readability-static-accessed-through-instance, -0.0)
+FAST(readability-static-definition-in-anonymous-namespace, 0.0)
+FAST(readability-string-compare, 1.0)
+FAST(readability-suspicious-call-argument, 1.0)
+FAST(readability-uniqueptr-delete-release, 1.0)
+FAST(readability-uppercase-literal-suffix, 1.0)
+FAST(readability-use-anyofallof, 1.0)
+FAST(zircon-temporary-objects, 0.0)
+
+#undef FAST
+#undef SLOW
+
diff --git a/clang-tools-extra/clangd/TidyFastChecks.py b/clang-tools-extra/clangd/TidyFastChecks.py
new file mode 100755
--- /dev/null
+++ b/clang-tools-extra/clangd/TidyFastChecks.py
@@ -0,0 +1,88 @@
+#!/usr/bin/env python3
+#
+# Determines which clang-tidy checks are "fast enough" to run in clangd.
+# This runs clangd --check --check-tidy-time and parses the output.
+# This program outputs a header fragment specifying which checks are fast:
+#   FAST(bugprone-argument-comment, 5)
+#   SLOW(misc-const-correctness, 200)
+# If given the old header fragment as input, we lean to preserve its choices.
+#
+# This is not deterministic or hermetic, but should be run occasionally to
+# update the list of allowed checks. From llvm-project:
+#   clang-tools-extra/clangd/TidyFastChecks.py \
+#     --clangd=build-opt/bin/clangd \
+#     > clang-tools-extra/clangd/TidyFastChecks.inc
+# Be sure to use an optimized, no-asserts, tidy-enabled build of clangd!
+
+import argparse
+import re
+import subprocess
+import sys
+
+# Checks faster than FAST_THRESHOLD are fast, slower than SLOW_THRESHOLD slow.
+# If a check is in between, we stick with our previous decision. This avoids
+# enabling/disabling checks between releases due to random measurement jitter.
+FAST_THRESHOLD = 8 # percent
+SLOW_THRESHOLD = 15
+
+parser = argparse.ArgumentParser()
+parser.add_argument('--old', help='Existing TidyFastChecks.inc, for hysteresis',
+    default='clang-tools-extra/clangd/TidyFastChecks.inc')
+parser.add_argument('--file', help='clangd binary to invoke',
+    default='clang/lib/Sema/Sema.cpp')
+parser.add_argument('--clangd', help='clangd binary to invoke',
+    default='build/bin/clangd')
+parser.add_argument('--checks', help='check glob to run', default='*')
+parser.add_argument('--verbose', help='log clangd output', action='store_true')
+args = parser.parse_args()
+
+# Use the preprocessor to extract the list of previously-fast checks.
+def read_old_fast(path):
+    text = subprocess.check_output(["cpp", "-P", "-FAST(C,T)=C", path])
+    for line in text.splitlines():
+        if line.strip():
+            yield line.strip().decode('utf-8')
+old_fast = list(read_old_fast(args.old)) if args.old else []
+print(f"Old fast checks: {old_fast}", file=sys.stderr)
+
+# Runs clangd --check --check-tidy-time.
+# Yields (check, percent-overhead) pairs.
+def measure():
+    process = subprocess.Popen([args.clangd,
+        "--check=" + args.file, "--check-locations=0", "--check-tidy-time=" + args.checks],
+        stderr=subprocess.PIPE)
+    recording = False
+    for line in iter(process.stderr.readline, b""):
+        if args.verbose:
+            print("clangd> ", line, file=sys.stderr)
+        if not recording:
+            if b'Timing AST build with individual clang-tidy checks' in line:  
+               recording = True
+            continue
+        if b'Finished individual clang-tidy checks' in line:
+            return
+        match = re.search(rb'(\S+) = (\S+)%', line)
+        if match:
+            yield (match.group(1).decode('utf-8'), float(match.group(2)))
+
+# Produce an includable X-macros fragment with our decisions.
+print("""// This file is generated, do not edit it directly!
+// This describes 
+#ifndef FAST
+#define FAST(CHECK, DELTA)
+#endif
+#ifndef SLOW
+#define SLOW(CHECK, DELTA)
+#endif
+""")
+
+for check, time in measure():
+    threshold = SLOW_THRESHOLD if check in old_fast else FAST_THRESHOLD
+    decision = "FAST" if time <= threshold else "SLOW"
+    print(f"{decision} {check} {time}% <= {threshold}%", file=sys.stderr)
+    print(f"{decision}({check}, {time})")
+
+print("""
+#undef FAST
+#undef SLOW
+""")
diff --git a/clang-tools-extra/clangd/tool/Check.cpp b/clang-tools-extra/clangd/tool/Check.cpp
--- a/clang-tools-extra/clangd/tool/Check.cpp
+++ b/clang-tools-extra/clangd/tool/Check.cpp
@@ -241,6 +241,9 @@
         elog("-{0} requires -DCLANGD_TIDY_CHECKS!", CheckTidyTime.ArgStr);
         return false;
       }
+      #ifndef NDEBUG
+      elog("Timing clang-tidy checks in asserts-mode is not representative!");
+      #endif
       checkTidyTimes();
     }