Index: clangd/TUScheduler.cpp
===================================================================
--- clangd/TUScheduler.cpp
+++ clangd/TUScheduler.cpp
@@ -383,6 +383,14 @@
     // Remove the old AST if it's still in cache.
     IdleASTs.take(this);
 
+    if (WantDiags == WantDiagnostics::No) {
+      // If no diagnostics were requested, building it in advance might be a
+      // waste if it will be removed from the cache before it is first accessed.
+      // So we choose to avoid building it in the first place, the rebuild will
+      // happen on first access instead.
+      return;
+    }
+
     // Build the AST for diagnostics.
     llvm::Optional<ParsedAST> AST =
         buildAST(FileName, std::move(Invocation), Inputs, NewPreamble, PCHs);