refactor: move slow performance tests to extra test suite

docs/test-optimization-summary.md

@@ -116,8 +116,67 @@ Based on the initial analysis (`mix test --slowest 20`), the following test file
 1. ✅ **Done:** Optimize seeds tests (this document)
 2. ⏳ **Next:** Review and optimize LiveView tests with large data setups
 3. ⏳ **Next:** Implement shared fixtures for policy tests
-4. ⏳ **Next:** Tag performance tests as `:slow` for conditional execution
-5. ⏳ **Future:** Consider nightly integration test suite for comprehensive coverage
+4. ✅ **Done:** Tag performance tests as `:slow` for conditional execution
+5. ✅ **Done:** Nightly integration test suite for comprehensive coverage
+
+---
+
+## Slow Test Suite
+
+Performance tests have been tagged with `@tag :slow` and separated into a dedicated test suite.
+
+### Structure
+
+- **Performance Tests:** Explicit tests that validate performance characteristics (e.g., N+1 query prevention, filter performance with large datasets)
+- **Tagging:** All performance tests are tagged with `@tag :slow` or `@moduletag :slow`
+- **Execution:** Standard test runs exclude slow tests, but they can be executed on demand
+
+### Execution
+
+**Fast Tests (Default):**
+```bash
+just test-fast
+# or
+mix test --exclude slow
+
+# With specific files or options
+just test-fast test/membership/member_test.exs
+just test-fast --seed 123
+```
+
+**Performance Tests Only:**
+```bash
+just test-slow
+# or
+mix test --only slow
+
+# With specific files or options
+just test-slow test/mv_web/member_live/index_test.exs
+just test-slow --seed 123
+```
+
+**All Tests:**
+```bash
+just test
+# or
+mix test
+
+# With specific files or options
+just test-all test/mv_web/
+just test-all --max-failures 5
+```
+
+**Note:** All suite commands (`test-fast`, `test-slow`, `test-all`) support additional arguments. The suite semantics (tags) are always preserved - additional arguments are appended to the command.
+
+### CI/CD Integration
+
+- **Standard CI:** Runs `mix test --exclude slow` for faster feedback loops
+- **Nightly Builds:** Separate pipeline (`nightly-tests`) runs daily at 2 AM and executes all performance tests
+- **Manual Execution:** Performance tests can be executed anytime with `just test-slow`
+
+### Further Details
+
+See [test-slow-suite.md](test-slow-suite.md) for complete documentation of the Slow Test Suite.
 
 ---
 
@@ -148,6 +207,7 @@ Based on the initial analysis (`mix test --slowest 20`), the following test file
 ## References
 
 - **Detailed Documentation:** `docs/test-optimization-seeds.md`
+- **Slow Test Suite:** `docs/test-slow-suite.md`
 - **Test File:** `test/seeds_test.exs`
 - **Original Analysis:** Internal benchmarking session (2026-01-28)
 - **Related Guidelines:** `CODE_GUIDELINES.md` - Section 4 (Testing Standards)