diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -283,15 +283,21 @@
 
   llvm::Optional<PreamblePatch> Patch;
   bool PreserveDiags = true;
+  // We might use an ignoring diagnostic consumer if they are going to be
+  // dropped later on to not pay for extra latency by processing them.
+  DiagnosticConsumer *DiagConsumer = &ASTDiags;
+  IgnoreDiagnostics DropDiags;
   if (Preamble) {
     Patch = PreamblePatch::createFullPatch(Filename, Inputs, *Preamble);
     Patch->apply(*CI);
     PreserveDiags = Patch->preserveDiagnostics();
+    if (!PreserveDiags)
+      DiagConsumer = &DropDiags;
   }
   auto Clang = prepareCompilerInstance(
       std::move(CI), PreamblePCH,
       llvm::MemoryBuffer::getMemBufferCopy(Inputs.Contents, Filename), VFS,
-      ASTDiags);
+      *DiagConsumer);
   if (!Clang) {
     // The last diagnostic contains information about the reason of this
     // failure.
@@ -301,6 +307,10 @@
                         : "unknown error");
     return None;
   }
+  if (!PreserveDiags) {
+    // Skips some analysis.
+    Clang->getDiagnosticOpts().IgnoreWarnings = true;
+  }
 
   auto Action = std::make_unique<ClangdFrontendAction>();
   const FrontendInputFile &MainInput = Clang->getFrontendOpts().Inputs[0];
@@ -329,7 +339,10 @@
   std::vector<std::unique_ptr<tidy::ClangTidyCheck>> CTChecks;
   ast_matchers::MatchFinder CTFinder;
   llvm::Optional<tidy::ClangTidyContext> CTContext;
-  {
+  llvm::Optional<IncludeFixer> FixIncludes;
+  // No need to run clang-tidy or IncludeFixerif we are not going to surface
+  // diagnostics.
+  if (PreserveDiags) {
     trace::Span Tracer("ClangTidyInit");
     tidy::ClangTidyOptions ClangTidyOpts =
         getTidyOptionsForFile(Inputs.ClangTidyProvider, Filename);
@@ -389,28 +402,28 @@
       }
       return DiagLevel;
     });
-  }
 
-  // Add IncludeFixer which can recover diagnostics caused by missing includes
-  // (e.g. incomplete type) and attach include insertion fixes to diagnostics.
-  llvm::Optional<IncludeFixer> FixIncludes;
-  auto BuildDir = VFS->getCurrentWorkingDirectory();
-  if (Inputs.Index && !BuildDir.getError()) {
-    auto Style = getFormatStyleForFile(Filename, Inputs.Contents, *Inputs.TFS);
-    auto Inserter = std::make_shared<IncludeInserter>(
-        Filename, Inputs.Contents, Style, BuildDir.get(),
-        &Clang->getPreprocessor().getHeaderSearchInfo());
-    if (Preamble) {
-      for (const auto &Inc : Preamble->Includes.MainFileIncludes)
-        Inserter->addExisting(Inc);
+    // Add IncludeFixer which can recover diagnostics caused by missing includes
+    // (e.g. incomplete type) and attach include insertion fixes to diagnostics.
+    auto BuildDir = VFS->getCurrentWorkingDirectory();
+    if (Inputs.Index && !BuildDir.getError()) {
+      auto Style =
+          getFormatStyleForFile(Filename, Inputs.Contents, *Inputs.TFS);
+      auto Inserter = std::make_shared<IncludeInserter>(
+          Filename, Inputs.Contents, Style, BuildDir.get(),
+          &Clang->getPreprocessor().getHeaderSearchInfo());
+      if (Preamble) {
+        for (const auto &Inc : Preamble->Includes.MainFileIncludes)
+          Inserter->addExisting(Inc);
+      }
+      FixIncludes.emplace(Filename, Inserter, *Inputs.Index,
+                          /*IndexRequestLimit=*/5);
+      ASTDiags.contributeFixes([&FixIncludes](DiagnosticsEngine::Level DiagLevl,
+                                              const clang::Diagnostic &Info) {
+        return FixIncludes->fix(DiagLevl, Info);
+      });
+      Clang->setExternalSemaSource(FixIncludes->unresolvedNameRecorder());
     }
-    FixIncludes.emplace(Filename, Inserter, *Inputs.Index,
-                        /*IndexRequestLimit=*/5);
-    ASTDiags.contributeFixes([&FixIncludes](DiagnosticsEngine::Level DiagLevl,
-                                            const clang::Diagnostic &Info) {
-      return FixIncludes->fix(DiagLevl, Info);
-    });
-    Clang->setExternalSemaSource(FixIncludes->unresolvedNameRecorder());
   }
 
   IncludeStructure Includes;
diff --git a/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp b/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
--- a/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
+++ b/clang-tools-extra/clangd/unittests/ParsedASTTests.cpp
@@ -460,77 +460,6 @@
         Code.substr(FileRange.Begin - 1, FileRange.End - FileRange.Begin + 2));
     EXPECT_EQ(SkippedFiles[I].kind(), tok::header_name);
   }
-
-  TU.AdditionalFiles["a.h"] = "";
-  TU.AdditionalFiles["b.h"] = "";
-  TU.AdditionalFiles["c.h"] = "";
-  // Make sure replay logic works with patched preambles.
-  llvm::StringLiteral Baseline = R"cpp(
-    #include "a.h"
-    #include "c.h")cpp";
-  MockFS FS;
-  TU.Code = Baseline.str();
-  auto Inputs = TU.inputs(FS);
-  auto BaselinePreamble = TU.preamble();
-  ASSERT_TRUE(BaselinePreamble);
-
-  // First make sure we don't crash on various modifications to the preamble.
-  llvm::StringLiteral Cases[] = {
-      // clang-format off
-      // New include in middle.
-      R"cpp(
-        #include "a.h"
-        #include "b.h"
-        #include "c.h")cpp",
-      // New include at top.
-      R"cpp(
-        #include "b.h"
-        #include "a.h"
-        #include "c.h")cpp",
-      // New include at bottom.
-      R"cpp(
-        #include "a.h"
-        #include "c.h"
-        #include "b.h")cpp",
-      // Same size with a missing include.
-      R"cpp(
-        #include "a.h"
-        #include "b.h")cpp",
-      // Smaller with no new includes.
-      R"cpp(
-        #include "a.h")cpp",
-      // Smaller with a new includes.
-      R"cpp(
-        #include "b.h")cpp",
-      // clang-format on
-  };
-  for (llvm::StringLiteral Case : Cases) {
-    TU.Code = Case.str();
-
-    IgnoreDiagnostics Diags;
-    auto CI = buildCompilerInvocation(TU.inputs(FS), Diags);
-    auto PatchedAST = ParsedAST::build(testPath(TU.Filename), TU.inputs(FS),
-                                       std::move(CI), {}, BaselinePreamble);
-    ASSERT_TRUE(PatchedAST);
-    EXPECT_FALSE(PatchedAST->getDiagnostics());
-  }
-
-  // Then ensure correctness by making sure includes were seen only once.
-  // Note that we first see the includes from the patch, as preamble includes
-  // are replayed after exiting the built-in file.
-  Includes.clear();
-  TU.Code = R"cpp(
-    #include "a.h"
-    #include "b.h")cpp";
-  IgnoreDiagnostics Diags;
-  auto CI = buildCompilerInvocation(TU.inputs(FS), Diags);
-  auto PatchedAST = ParsedAST::build(testPath(TU.Filename), TU.inputs(FS),
-                                     std::move(CI), {}, BaselinePreamble);
-  ASSERT_TRUE(PatchedAST);
-  EXPECT_FALSE(PatchedAST->getDiagnostics());
-  EXPECT_THAT(Includes,
-              ElementsAre(WithFileName(testPath("__preamble_patch__.h")),
-                          WithFileName("b.h"), WithFileName("a.h")));
 }
 
 TEST(ParsedASTTest, PatchesAdditionalIncludes) {