--- a/python/mozbuild/mozpack/copier.py
+++ b/python/mozbuild/mozpack/copier.py
@@ -1,28 +1,30 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import absolute_import
 
 import os
 import stat
+import sys
 
 from mozpack.errors import errors
 from mozpack.files import (
     BaseFile,
     Dest,
 )
 import mozpack.path as mozpath
 import errno
 from collections import (
     Counter,
     OrderedDict,
 )
+import concurrent.futures as futures
 
 
 class FileRegistry(object):
     '''
     Generic container to keep track of a set of BaseFile instances. It
     preserves the order under which the files are added, but doesn't keep
     track of empty directories (directories are not stored at all).
     The paths associated with the BaseFile instances are relative to an
@@ -370,20 +372,40 @@ class FileCopier(FileRegistry):
 
                 for f in files:
                     existing_files.add(os.path.normpath(os.path.join(root, f)))
 
         # Now we reconcile the state of the world against what we want.
         dest_files = set()
 
         # Install files.
-        for p, f in self:
-            destfile = os.path.normpath(os.path.join(destination, p))
+        #
+        # Creating/appending new files on Windows/NTFS is slow. So we use a
+        # thread pool to speed it up significantly. The performance of this
+        # loop is so critical to common build operations on Linux that the
+        # overhead of the thread pool is worth avoiding, so we have 2 code
+        # paths. We also employ a low water mark to prevent thread pool
+        # creation if number of files is too small to benefit.
+        copy_results = []
+        if sys.platform == 'win32' and len(self) > 100:
+            with futures.ThreadPoolExecutor(4) as e:
+                fs = []
+                for p, f in self:
+                    destfile = os.path.normpath(os.path.join(destination, p))
+                    fs.append((destfile, e.submit(f.copy, destfile, skip_if_older)))
+
+            copy_results = [(destfile, f.result) for destfile, f in fs]
+        else:
+            for p, f in self:
+                destfile = os.path.normpath(os.path.join(destination, p))
+                copy_results.append((destfile, f.copy(destfile, skip_if_older)))
+
+        for destfile, copy_result in copy_results:
             dest_files.add(destfile)
-            if f.copy(destfile, skip_if_older):
+            if copy_result:
                 result.updated_files.add(destfile)
             else:
                 result.existing_files.add(destfile)
 
         # Remove files no longer accounted for.
         if remove_unaccounted:
             for f in existing_files - dest_files:
                 # Windows requires write access to remove files.