Index: clangd/CodeComplete.cpp
===================================================================
--- clangd/CodeComplete.cpp
+++ clangd/CodeComplete.cpp
@@ -519,14 +519,11 @@
   std::unique_ptr<llvm::MemoryBuffer> ContentsBuffer =
       llvm::MemoryBuffer::getMemBufferCopy(Contents, FileName);
 
-  // Attempt to reuse the PCH from precompiled preamble, if it was built.
-  if (Preamble) {
-    auto Bounds =
-        ComputePreambleBounds(*CI->getLangOpts(), ContentsBuffer.get(), 0);
-    if (!Preamble->CanReuse(*CI, ContentsBuffer.get(), Bounds, VFS.get()))
-      Preamble = nullptr;
-  }
-
+  // Note that we delibirately don't check if preamble is up-to-date. This
+  // operation is very expensive and we feel the right trade-off here is to
+  // trade correctness in some cases (preamble has not been rebuilt after
+  // changes to a file) for performance. Eventually the preamble will be rebuilt
+  // and code completion will produce correct results.
   auto Clang = prepareCompilerInstance(
       std::move(CI), Preamble, std::move(ContentsBuffer), std::move(PCHs),
       std::move(VFS), DummyDiagsConsumer);
Index: clangd/Compiler.h
===================================================================
--- clangd/Compiler.h
+++ clangd/Compiler.h
@@ -29,12 +29,14 @@
 };
 
 /// Creates a CompilerInstance with the main file contens overridden.
-/// The preamble will be reused unless it is null.
-/// Note that the vfs::FileSystem inside returned instance may differ if
-/// additional file remappings occur in command-line arguments.
-/// On some errors, returns null. When non-null value is returned, it's expected
-/// to be consumed by the FrontendAction as it will have a pointer to the
-/// MainFile buffer that will only be deleted if BeginSourceFile is called.
+/// The preamble will be set up via PrecompiledPreamble::OverridePreamble
+/// without doing any extra checks, the callers are responsible for checking
+/// PrecompiledPreamble::CanReuse if they need to make sure only up-to-date
+/// preambles are used . Note that the vfs::FileSystem inside returned instance
+/// may differ if additional file remappings occur in command-line arguments. On
+/// some errors, returns null. When non-null value is returned, it's expected to
+/// be consumed by the FrontendAction as it will have a pointer to the MainFile
+/// buffer that will only be deleted if BeginSourceFile is called.
 std::unique_ptr<CompilerInstance> prepareCompilerInstance(
     std::unique_ptr<clang::CompilerInvocation>, const PrecompiledPreamble *,
     std::unique_ptr<llvm::MemoryBuffer> MainFile,
Index: clangd/Compiler.cpp
===================================================================
--- clangd/Compiler.cpp
+++ clangd/Compiler.cpp
@@ -36,7 +36,7 @@
   // NOTE: we use Buffer.get() when adding remapped files, so we have to make
   // sure it will be released if no error is emitted.
   if (Preamble) {
-    Preamble->AddImplicitPreamble(*CI, VFS, Buffer.get());
+    Preamble->OverridePreamble(*CI, VFS, Buffer.get());
   } else {
     CI->getPreprocessorOpts().addRemappedFile(
         CI->getFrontendOpts().Inputs[0].getFile(), Buffer.get());