diff --git a/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp b/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
--- a/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
@@ -386,9 +386,9 @@
 
     using flags::FLAGS_FOO;
 
-    // CLI will come from a define, __llvm__ is a built-in. In both cases, they
+    // CLI will come from a define, __cplusplus is a built-in. In both cases, they
     // come from non-existent files.
-    int y = CLI + __llvm__;
+    int y = CLI + __cplusplus;
 
     int concat(a, b) = 42;
     )cpp";
diff --git a/clang-tools-extra/clangd/unittests/TestTU.cpp b/clang-tools-extra/clangd/unittests/TestTU.cpp
--- a/clang-tools-extra/clangd/unittests/TestTU.cpp
+++ b/clang-tools-extra/clangd/unittests/TestTU.cpp
@@ -40,7 +40,9 @@
   ParseInputs Inputs;
   Inputs.FeatureModules = FeatureModules;
   auto &Argv = Inputs.CompileCommand.CommandLine;
-  Argv = {"clang"};
+  // In tests, omit predefined macros (__GNUC__ etc) for a 25% speedup.
+  // There are hundreds, and we'd generate, parse, serialize, and re-parse them!
+  Argv = {"clang", "-Xclang", "-undef"};
   // FIXME: this shouldn't need to be conditional, but it breaks a
   // GoToDefinition test for some reason (getMacroArgExpandedLocation fails).
   if (!HeaderCode.empty()) {