refactor: move slow performance tests to extra test suite

CODE_GUIDELINES.md

@@ -1654,6 +1654,28 @@ end
 - Mock external services
 - Use fixtures efficiently
 
+**Performance Tests:**
+
+Performance tests that explicitly validate performance characteristics should be tagged with `@tag :slow` or `@moduletag :slow` to exclude them from standard test runs. This improves developer feedback loops while maintaining comprehensive coverage.
+
+**When to Tag as `:slow`:**
+
+- Tests that explicitly measure execution time or validate performance characteristics
+- Tests that use large datasets (e.g., 50+ records) to test scalability
+- Tests that validate query optimization (N+1 prevention, index usage)
+
+**Running Tests:**
+
+```bash
+# Fast tests only (default)
+mix test --exclude slow
+
+# Performance tests only
+mix test --only slow
+
+# All tests
+mix test
+```
 ---
 
 ## 5. Security Guidelines