diff --git a/doc/build.py b/doc/build.py
index 40956571..dfa124f1 100755
--- a/doc/build.py
+++ b/doc/build.py
@@ -15,11 +15,11 @@ def pip_install(package, commit=None, **kwargs):
 
 def create_build_env(dirname='virtualenv'):
   # Create virtualenv.
-  check_call(['virtualenv', dirname])
+  if not os.path.exists(dirname):
+    check_call(['virtualenv', dirname])
   import sysconfig
   scripts_dir = os.path.basename(sysconfig.get_path('scripts'))
-  activate_this_file = os.path.join(dirname, scripts_dir,
-                                    'activate_this.py')
+  activate_this_file = os.path.join(dirname, scripts_dir, 'activate_this.py')
   with open(activate_this_file) as f:
     exec(f.read(), dict(__file__=activate_this_file))
   # Upgrade pip because installation of sphinx with pip 1.1 available on Travis
@@ -39,8 +39,7 @@ def create_build_env(dirname='virtualenv'):
   except DistributionNotFound:
     pass
   # Install Sphinx and Breathe.
-  pip_install('sphinx-doc/sphinx',
-              '12b83372ac9316e8cbe86e7fed889296a4cc29ee',
+  pip_install('sphinx-doc/sphinx', '12b83372ac9316e8cbe86e7fed889296a4cc29ee',
               check_version='1.4.1.dev20160525')
   pip_install('michaeljones/breathe',
               '6b1c5bb7a1866f15fc328b8716258354b10c1daa')
diff --git a/support/update-website.py b/support/update-website.py
index da90dea4..416f49be 100755
--- a/support/update-website.py
+++ b/support/update-website.py
@@ -26,12 +26,15 @@ class Git:
 fmt_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.insert(0, os.path.join(fmt_dir, 'doc'))
 
-workdir = tempfile.mkdtemp('fmt')
+work_dir = tempfile.mkdtemp('fmt')
 try:
   import build
-  build.create_build_env(os.path.join(workdir, 'virtualenv'))
+  # The virtualenv is stored in ~/.cache/fmt/virtualenv and reused to speed
+  # up builds.
+  build.create_build_env(os.path.join(
+    os.path.expanduser('~'), '.cache', 'fmt', 'virtualenv'))
 
-  fmt_repo = Git(os.path.join(workdir, 'fmt'))
+  fmt_repo = Git(os.path.join(work_dir, 'fmt'))
   fmt_repo.clone('git@github.com:fmtlib/fmt')
   doc_repo = Git('fmtlib.github.io')
   doc_repo.clone('git@github.com:fmtlib/fmtlib.github.io')
@@ -68,7 +71,8 @@ try:
       with open(reference, 'w') as f:
         f.write(data)
     # Build the docs.
-    build.build_docs(version, doc_dir=target_doc_dir, include_dir=fmt_repo.dir)
+    build.build_docs(version, doc_dir=target_doc_dir, include_dir=fmt_repo.dir,
+                     work_dir=work_dir)
     # Create symlinks for older versions.
     for link, target in {'index': 'contents', 'api': 'reference'}.items():
       os.symlink(target + '.html', os.path.join('html', link) + '.html')
@@ -76,8 +80,7 @@ try:
     version_doc_dir = os.path.join(doc_repo.dir, version)
     shutil.rmtree(version_doc_dir)
     shutil.move('html', version_doc_dir)
-    # TODO: generate 'html' in temp dir
     # TODO: update links in navbar
     # TODO: remove doc repo
 except:
-  shutil.rmtree(workdir)
+  shutil.rmtree(work_dir)