diff --git a/testing/conftest.py b/testing/conftest.py
index 4badf3016..6e01d710d 100644
--- a/testing/conftest.py
+++ b/testing/conftest.py
@@ -30,6 +30,7 @@ def pytest_collection_modifyitems(config, items):
                     slowest_items.append(item)
                 else:
                     slow_items.append(item)
+                item.add_marker(pytest.mark.slow)
             else:
                 marker = item.get_closest_marker("slow")
                 if marker: