Index: clangd/tool/ClangdMain.cpp
===================================================================
--- clangd/tool/ClangdMain.cpp
+++ clangd/tool/ClangdMain.cpp
@@ -259,6 +259,9 @@
   if (Tracer)
     TracingSession.emplace(*Tracer);
 
+  // Use buffered stream to stderr. Unbuffered stream can cause significant
+  // (non-deterministic) latency for the logger.
+  llvm::errs().SetBuffered();
   JSONOutput Out(llvm::outs(), llvm::errs(), LogLevel,
                  InputMirrorStream ? InputMirrorStream.getPointer() : nullptr,
                  PrettyPrint);