diff --git a/betty/fs.py b/betty/fs.py
index 12dfd3f89..03f196f28 100644
--- a/betty/fs.py
+++ b/betty/fs.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 import hashlib
 import os
 from collections import deque
@@ -12,7 +13,6 @@
 
 import aiofiles
 from aiofiles.os import makedirs
-from aiofiles.ospath import exists
 from aiofiles.threadpool.text import AsyncTextIOWrapper
 
 from betty import _ROOT_DIRECTORY_PATH
@@ -86,13 +86,12 @@ async def copy2(self, source_path: Path, destination_path: Path) -> Path:
         raise FileNotFoundError('Could not find any of %s.' % ', '.join(tried_paths))
 
     async def copytree(self, source_path: Path, destination_path: Path) -> AsyncIterable[Path]:
-        destination_paths = set()
+        file_destination_paths = set()
         for fs_path, _ in self._paths:
             async for file_source_path in iterfiles(fs_path / source_path):
                 file_destination_path = destination_path / file_source_path.relative_to(fs_path / source_path)
-                if not await exists(file_destination_path):
+                if file_destination_path not in file_destination_paths:
+                    file_destination_paths.add(file_destination_path)
                     await makedirs(file_destination_path.parent, exist_ok=True)
-                    copy2(file_source_path, file_destination_path)
-                    if file_destination_path not in destination_paths:
-                        destination_paths.add(file_destination_path)
-                        yield file_destination_path
+                    await asyncio.to_thread(copy2, file_source_path, file_destination_path)
+                    yield file_destination_path
diff --git a/betty/tests/test_generate.py b/betty/tests/test_generate.py
index 82200d449..729a56a11 100644
--- a/betty/tests/test_generate.py
+++ b/betty/tests/test_generate.py
@@ -182,7 +182,7 @@ async def test_source(self) -> None:
 class TestResourceOverride:
     async def test(self) -> None:
         async with App() as app:
-            localized_assets_directory_path = Path(app.project.configuration.assets_directory_path) / 'public' / 'static'
+            localized_assets_directory_path = Path(app.project.configuration.assets_directory_path) / 'public' / 'localized'
             localized_assets_directory_path.mkdir(parents=True)
             with open(str(localized_assets_directory_path / 'index.html.j2'), 'w') as f:
                 f.write('{% block page_content %}Betty was here{% endblock %}')