diff --git a/clang-tools-extra/clangd/Compiler.cpp b/clang-tools-extra/clangd/Compiler.cpp
--- a/clang-tools-extra/clangd/Compiler.cpp
+++ b/clang-tools-extra/clangd/Compiler.cpp
@@ -85,10 +85,16 @@
 std::unique_ptr<CompilerInvocation>
 buildCompilerInvocation(const ParseInputs &Inputs, clang::DiagnosticConsumer &D,
                         std::vector<std::string> *CC1Args) {
-  if (Inputs.CompileCommand.CommandLine.empty())
+  llvm::ArrayRef<std::string> Argv = Inputs.CompileCommand.CommandLine;
+  if (Argv.empty())
     return nullptr;
   std::vector<const char *> ArgStrs;
-  for (const auto &S : Inputs.CompileCommand.CommandLine)
+  ArgStrs.reserve(Argv.size() + 1);
+  // In asserts builds, CompilerInvocation redundantly reads/parses cc1 args as
+  // a sanity test. This is not useful to clangd, and costs 10% of test time.
+  // To avoid mismatches between assert/production builds, disable it always.
+  ArgStrs = {Argv.front().c_str(), "-Xclang", "-no-round-trip-args"};
+  for (const auto &S : Argv.drop_front())
     ArgStrs.push_back(S.c_str());
 
   CreateInvocationOptions CIOpts;