Index: clang-tools-extra/clangd/index/Background.cpp
===================================================================
--- clang-tools-extra/clangd/index/Background.cpp
+++ clang-tools-extra/clangd/index/Background.cpp
@@ -127,7 +127,7 @@
     BackgroundIndexStorage::Factory IndexStorageFactory, size_t ThreadPoolSize)
     : SwapIndex(llvm::make_unique<MemIndex>()), FSProvider(FSProvider),
       CDB(CDB), BackgroundContext(std::move(BackgroundContext)),
-      Rebuilder(this, &IndexedSymbols),
+      Rebuilder(this, &IndexedSymbols, ThreadPoolSize),
       IndexStorageFactory(std::move(IndexStorageFactory)),
       CommandsChanged(
           CDB.watch([&](const std::vector<std::string> &ChangedFiles) {
Index: clang-tools-extra/clangd/index/BackgroundRebuild.h
===================================================================
--- clang-tools-extra/clangd/index/BackgroundRebuild.h
+++ clang-tools-extra/clangd/index/BackgroundRebuild.h
@@ -16,6 +16,7 @@
 
 #include "index/FileIndex.h"
 #include "index/Index.h"
+#include "llvm/Support/Threading.h"
 
 namespace clang {
 namespace clangd {
@@ -45,12 +46,10 @@
 // This class is exposed in the header so it can be tested.
 class BackgroundIndexRebuilder {
 public:
-  // Thresholds for rebuilding as TUs get indexed.
-  static constexpr unsigned TUsBeforeFirstBuild = 5;
-  static constexpr unsigned TUsBeforeRebuild = 100;
-
-  BackgroundIndexRebuilder(SwapIndex *Target, FileSymbols *Source)
-      : Target(Target), Source(Source) {}
+  BackgroundIndexRebuilder(
+      SwapIndex *Target, FileSymbols *Source,
+      unsigned Threads = llvm::heavyweight_hardware_concurrency())
+      : TUsBeforeFirstBuild(Threads), Target(Target), Source(Source) {}
 
   // Called to indicate a TU has been indexed.
   // May rebuild, if enough TUs have been indexed.
@@ -71,6 +70,10 @@
   // Ensures we won't start any more rebuilds.
   void shutdown();
 
+  // Thresholds for rebuilding as TUs get indexed.
+  const unsigned TUsBeforeFirstBuild; // Typically one per worker thread.
+  const unsigned TUsBeforeRebuild = 100;
+
 private:
   // Run Check under the lock, and rebuild if it returns true.
   void maybeRebuild(const char *Reason, std::function<bool()> Check);
Index: clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
===================================================================
--- clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
+++ clang-tools-extra/clangd/unittests/BackgroundIndexTests.cpp
@@ -610,11 +610,10 @@
 };
 
 TEST_F(BackgroundIndexRebuilderTest, IndexingTUs) {
-  for (unsigned I = 0; I < BackgroundIndexRebuilder::TUsBeforeFirstBuild - 1;
-       ++I)
+  for (unsigned I = 0; I < Rebuilder.TUsBeforeFirstBuild - 1; ++I)
     EXPECT_FALSE(checkRebuild([&] { Rebuilder.indexedTU(); }));
   EXPECT_TRUE(checkRebuild([&] { Rebuilder.indexedTU(); }));
-  for (unsigned I = 0; I < BackgroundIndexRebuilder::TUsBeforeRebuild - 1; ++I)
+  for (unsigned I = 0; I < Rebuilder.TUsBeforeRebuild - 1; ++I)
     EXPECT_FALSE(checkRebuild([&] { Rebuilder.indexedTU(); }));
   EXPECT_TRUE(checkRebuild([&] { Rebuilder.indexedTU(); }));
 }
@@ -640,7 +639,7 @@
 
   // No rebuilding for indexed files while loading.
   Rebuilder.startLoading();
-  for (unsigned I = 0; I < 3 * BackgroundIndexRebuilder::TUsBeforeRebuild; ++I)
+  for (unsigned I = 0; I < 3 * Rebuilder.TUsBeforeRebuild; ++I)
     EXPECT_FALSE(checkRebuild([&] { Rebuilder.indexedTU(); }));
   // But they get indexed when we're done, even if no shards were loaded.
   EXPECT_TRUE(checkRebuild([&] { Rebuilder.doneLoading(); }));