diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -29,6 +29,7 @@
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/TimeProfiler.h"
 #include "llvm/Support/xxhash.h"
 
@@ -64,6 +65,7 @@
 
   template <class LP> void run();
 
+  ThreadPool writerThreadPool;
   std::unique_ptr<FileOutputBuffer> &buffer;
   uint64_t addr = 0;
   uint64_t fileOff = 0;
@@ -1147,8 +1149,8 @@
   sortSegmentsAndSections();
   createLoadCommands<LP>();
   finalizeAddresses();
+  writerThreadPool.async(writeMapFile);
   finalizeLinkEditSegment();
-  writeMapFile();
   writeOutputFile();
 }