diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -61,21 +61,35 @@
 struct UpdateIndexCallbacks : public ParsingCallbacks {
   UpdateIndexCallbacks(FileIndex *FIndex,
                        ClangdServer::Callbacks *ServerCallbacks,
-                       const ThreadsafeFS &TFS, AsyncTaskRunner *Tasks)
+                       const ThreadsafeFS &TFS, AsyncTaskRunner *Tasks,
+                       AsyncTaskRunner* PreambleIndexTask)
       : FIndex(FIndex), ServerCallbacks(ServerCallbacks), TFS(TFS),
-        Stdlib{std::make_shared<StdLibSet>()}, Tasks(Tasks) {}
+        Stdlib{std::make_shared<StdLibSet>()}, Tasks(Tasks),
+        PreambleIndexTask(PreambleIndexTask) {}
 
   void onPreambleAST(PathRef Path, llvm::StringRef Version,
                      const CompilerInvocation &CI, ASTContext &Ctx,
                      Preprocessor &PP,
                      const CanonicalIncludes &CanonIncludes) override {
+    if (!FIndex)
+      return;
+
     // If this preamble uses a standard library we haven't seen yet, index it.
-    if (FIndex)
-      if (auto Loc = Stdlib->add(*CI.getLangOpts(), PP.getHeaderSearchInfo()))
-        indexStdlib(CI, std::move(*Loc));
+    if (auto Loc = Stdlib->add(*CI.getLangOpts(), PP.getHeaderSearchInfo()))
+      indexStdlib(CI, std::move(*Loc));
 
-    if (FIndex)
-      FIndex->updatePreamble(Path, Version, Ctx, PP, CanonIncludes);
+    auto Task = [FIndex(FIndex), Path(Path),
+                Version(Version), Ctx(&Ctx), PP(&PP),
+                CanonIncludes(CanonIncludes)] {
+      FIndex->updatePreamble(Path, Version, *Ctx, *PP, CanonIncludes);
+    };
+
+    assert(PreambleIndexTask);
+    if (PreambleIndexTask)
+      PreambleIndexTask->runAsync("task:" + Path + Version,
+                                  std::move(Task));
+    else
+      Task();
   }
 
   void indexStdlib(const CompilerInvocation &CI, StdLibLocation Loc) {
@@ -139,6 +153,7 @@
   const ThreadsafeFS &TFS;
   std::shared_ptr<StdLibSet> Stdlib;
   AsyncTaskRunner *Tasks;
+  AsyncTaskRunner *PreambleIndexTask;
 };
 
 class DraftStoreFS : public ThreadsafeFS {
@@ -201,7 +216,8 @@
   WorkScheduler.emplace(CDB, TUScheduler::Options(Opts),
                         std::make_unique<UpdateIndexCallbacks>(
                             DynamicIdx.get(), Callbacks, TFS,
-                            IndexTasks ? &*IndexTasks : nullptr));
+                            IndexTasks ? &*IndexTasks : nullptr,
+                             WorkScheduler->getPreambleIndexTasks()));
   // Adds an index to the stack, at higher priority than existing indexes.
   auto AddIndex = [&](SymbolIndex *Idx) {
     if (this->Index != nullptr) {
diff --git a/clang-tools-extra/clangd/TUScheduler.h b/clang-tools-extra/clangd/TUScheduler.h
--- a/clang-tools-extra/clangd/TUScheduler.h
+++ b/clang-tools-extra/clangd/TUScheduler.h
@@ -335,6 +335,10 @@
   /// Mostly useful for synchronizing tests.
   bool blockUntilIdle(Deadline D) const;
 
+  AsyncTaskRunner* getPreambleIndexTasks() {
+    return PreambleIndexTasks ? &PreambleIndexTasks.value() : nullptr;
+  }
+
 private:
   /// This class stores per-file data in the Files map.
   struct FileData;
@@ -371,6 +375,7 @@
   // None when running tasks synchronously and non-None when running tasks
   // asynchronously.
   std::optional<AsyncTaskRunner> PreambleTasks;
+  std::optional<AsyncTaskRunner> PreambleIndexTasks;
   std::optional<AsyncTaskRunner> WorkerThreads;
   // Used to create contexts for operations that are not bound to a particular
   // file (e.g. index queries).
diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -1646,6 +1646,7 @@
   }
   if (0 < Opts.AsyncThreadsCount) {
     PreambleTasks.emplace();
+    PreambleIndexTasks.emplace();
     WorkerThreads.emplace();
   }
 }
@@ -1657,6 +1658,8 @@
   // Wait for all in-flight tasks to finish.
   if (PreambleTasks)
     PreambleTasks->wait();
+  if (PreambleIndexTasks)
+    PreambleIndexTasks->wait();
   if (WorkerThreads)
     WorkerThreads->wait();
 }
@@ -1668,6 +1671,9 @@
   if (PreambleTasks)
     if (!PreambleTasks->wait(D))
       return false;
+  if (PreambleIndexTasks)
+    if (!PreambleIndexTasks->wait(D))
+      return false;
   return true;
 }