Skip performance tests by default

- Add 'performance' marker to pytest.ini
- Add pytestmark to test_cache_performance.py
- Use -m 'not performance' by default in addopts
- Allows manual execution with 'pytest -m performance'

pytest.ini

@@ -1,5 +1,5 @@
 [pytest]
-addopts = -v --import-mode=importlib
+addopts = -v --import-mode=importlib -m "not performance"
 testpaths = tests
 python_files = test_*.py
 python_classes = Test*
@@ -12,5 +12,6 @@ markers =
     asyncio: execute test within asyncio event loop
     no_settings_dir_isolation: allow tests to use real settings paths
     integration: integration tests requiring external resources
+    performance: performance benchmarks (slow, skip by default)
 # Skip problematic directories to avoid import conflicts
 norecursedirs = .git .tox dist build *.egg __pycache__ py .hypothesis