diff --git a/.drone.yml b/.drone.yml
index e207ca0..ebc74e7 100644
--- a/.drone.yml
+++ b/.drone.yml
@@ -83,8 +83,8 @@ steps:
       - mix local.hex --force
       # Fetch dependencies
       - mix deps.get
-      # Run tests
-      - mix test
+      # Run fast tests (excludes slow/performance tests)
+      - mix test --exclude slow
 
   - name: rebuild-cache
     image: drillster/drone-volume-cache
@@ -178,3 +178,52 @@ steps:
       - unset GIT_COMMITTER_NAME GIT_COMMITTER_EMAIL GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL
       - renovate-config-validator
       - renovate
+
+---
+kind: pipeline
+type: docker
+name: nightly-tests
+
+trigger:
+  event:
+    - cron
+  cron:
+    - "0 2 * * *"  # Run at 2 AM daily
+
+services:
+  - name: postgres
+    image: docker.io/library/postgres:18.1
+    environment:
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+
+steps:
+  - name: wait_for_postgres
+    image: docker.io/library/postgres:18.1
+    commands:
+      # Wait for postgres to become available
+      - |
+        for i in {1..20}; do
+          if pg_isready -h postgres -U postgres; then
+            exit 0
+          else
+            true
+          fi
+          sleep 2
+        done
+        echo "Postgres did not become available, aborting."
+        exit 1
+
+  - name: test-slow
+    image: docker.io/library/elixir:1.18.3-otp-27
+    environment:
+      MIX_ENV: test
+      TEST_POSTGRES_HOST: postgres
+      TEST_POSTGRES_PORT: 5432
+    commands:
+      # Install hex package manager
+      - mix local.hex --force
+      # Fetch dependencies
+      - mix deps.get
+      # Run only all tests
+      - mix test
diff --git a/CODE_GUIDELINES.md b/CODE_GUIDELINES.md
index 28c454b..b11a369 100644
--- a/CODE_GUIDELINES.md
+++ b/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
diff --git a/Justfile b/Justfile
index f25041c..6337d7c 100644
--- a/Justfile
+++ b/Justfile
@@ -22,7 +22,7 @@ seed-database:
 start-database:
     docker compose up -d
 
-ci-dev: lint audit test
+ci-dev: lint audit test-fast
 
 gettext:
   mix gettext.extract
@@ -44,6 +44,18 @@ audit:
 test *args: install-dependencies
     mix test {{args}}
 
+# Run only fast tests (excludes slow/performance tests)
+test-fast *args: install-dependencies
+    mix test --exclude slow {{args}}
+
+# Run only slow/performance tests
+test-slow *args: install-dependencies
+    mix test --only slow {{args}}
+
+# Run all tests (fast + slow)
+test-all *args: install-dependencies
+    mix test {{args}}
+
 format:
     mix format
 
diff --git a/docs/test-optimization-summary.md b/docs/test-optimization-summary.md
index ae50081..001a235 100644
--- a/docs/test-optimization-summary.md
+++ b/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)
diff --git a/docs/test-slow-suite.md b/docs/test-slow-suite.md
new file mode 100644
index 0000000..e4cfcc2
--- /dev/null
+++ b/docs/test-slow-suite.md
@@ -0,0 +1,280 @@
+# Slow Test Suite Documentation
+
+**Date:** 2026-01-28  
+**Status:** ✅ Active
+
+---
+
+## Overview
+
+The Slow Test Suite contains performance tests that explicitly validate performance characteristics of the application. These tests are intentionally slower because they use larger datasets or test performance-critical paths (e.g., N+1 query prevention, filter performance with many records).
+
+These tests are marked with `@tag :slow` or `@moduletag :slow` and are excluded from standard test runs to improve developer feedback loops while maintaining comprehensive coverage.
+
+---
+
+## Purpose
+
+Performance tests serve to:
+
+1. **Validate Performance Characteristics:** Ensure queries and operations perform within acceptable time limits
+2. **Prevent Regressions:** Catch performance regressions before they reach production
+3. **Test Scalability:** Verify that the application handles larger datasets efficiently
+4. **N+1 Query Prevention:** Ensure proper preloading and query optimization
+
+---
+
+## Identified Performance Tests
+
+### 1. Member LiveView - Boolean Filter Performance
+
+**File:** `test/mv_web/member_live/index_test.exs`  
+**Test:** `"boolean filter performance with 150 members"`  
+**Duration:** ~3.8 seconds  
+**Purpose:** Validates that boolean custom field filtering performs efficiently with 150 members
+
+**What it tests:**
+- Creates 150 members (75 with `true`, 75 with `false` for a boolean custom field)
+- Tests filter performance (< 1 second requirement)
+- Verifies correct filtering behavior
+
+### 2. Group LiveView - Index Performance
+
+**File:** `test/mv_web/live/group_live/index_test.exs`  
+**Describe Block:** `"performance"`  
+**Tests:** 2 tests  
+**Purpose:** Validates efficient page loading and member count calculation
+
+**What it tests:**
+- Page loads efficiently with many groups (no N+1 queries)
+- Member count calculation is efficient
+
+### 3. Group LiveView - Show Performance
+
+**File:** `test/mv_web/live/group_live/show_test.exs`  
+**Describe Block:** `"performance"`  
+**Tests:** 2 tests  
+**Purpose:** Validates efficient member list loading and slug lookup
+
+**What it tests:**
+- Member list is loaded efficiently (no N+1 queries)
+- Slug lookup uses unique_slug index efficiently
+
+### 4. Member LiveView - Membership Fee Status Performance
+
+**File:** `test/mv_web/member_live/index_membership_fee_status_test.exs`  
+**Describe Block:** `"performance"`  
+**Tests:** 1 test  
+**Purpose:** Validates efficient cycle loading without N+1 queries
+
+**What it tests:**
+- Cycles are loaded efficiently without N+1 queries
+- Multiple members with cycles render without performance issues
+
+### 5. Group Integration - Query Performance
+
+**File:** `test/membership/group_integration_test.exs`  
+**Describe Block:** `"Query Performance"`  
+**Tests:** 1 test  
+**Purpose:** Validates N+1 query prevention for group-member relationships
+
+**What it tests:**
+- Preloading groups with members avoids N+1 queries
+- Query optimization for many-to-many relationships
+
+---
+
+## Running Slow Tests
+
+### Local Development
+
+**Run only 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
+```
+
+**Run only performance tests:**
+```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
+```
+
+**Run all tests (fast + slow):**
+```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
+
+**Standard CI Pipeline:**
+- Runs `mix test --exclude slow` for faster feedback
+- Executes on every push to any branch
+
+**Nightly Pipeline:**
+- Runs `mix test --only slow` for comprehensive performance coverage
+- Executes daily at 2 AM via cron trigger
+- Pipeline name: `nightly-tests`
+
+---
+
+## Best Practices for New Performance Tests
+
+### When to Tag as `:slow`
+
+Tag tests as `:slow` when they:
+
+1. **Explicitly test performance:** Tests that measure execution time or validate performance characteristics
+2. **Use large datasets:** Tests that create many records (e.g., 50+ members, 100+ records)
+3. **Test scalability:** Tests that verify the application handles larger workloads
+4. **Validate query optimization:** Tests that ensure N+1 queries are prevented
+
+### When NOT to Tag as `:slow`
+
+Do NOT tag tests as `:slow` if they are:
+
+1. **Simply slow by accident:** Tests that are slow due to inefficient setup, not intentional performance testing
+2. **Slow due to bugs:** Tests that are slow because of actual performance bugs (fix the bug instead)
+3. **Integration tests:** Integration tests should be tagged separately if needed (`@tag :integration`)
+
+### Tagging Guidelines
+
+**For single tests:**
+```elixir
+@tag :slow
+test "boolean filter performance with 150 members" do
+  # test implementation
+end
+```
+
+**For describe blocks (all tests in block):**
+```elixir
+@moduletag :slow
+describe "performance" do
+  test "page loads efficiently" do
+    # test implementation
+  end
+  
+  test "member count is efficient" do
+    # test implementation
+  end
+end
+```
+
+---
+
+## Performance Test Structure
+
+### Recommended Structure
+
+```elixir
+defmodule MvWeb.SomeLiveViewTest do
+  use MvWeb.ConnCase, async: true
+  
+  # Regular tests here (not tagged)
+  
+  @moduletag :slow
+  describe "performance" do
+    test "loads efficiently without N+1 queries" do
+      # Create test data
+      # Measure/validate performance
+      # Assert correct behavior
+    end
+    
+    test "handles large datasets efficiently" do
+      # Create large dataset
+      # Measure performance
+      # Assert performance requirements
+    end
+  end
+end
+```
+
+### Performance Assertions
+
+Performance tests should include explicit performance assertions:
+
+```elixir
+# Example: Time-based assertion
+start_time = System.monotonic_time(:millisecond)
+# ... perform operation ...
+end_time = System.monotonic_time(:millisecond)
+duration = end_time - start_time
+
+assert duration < 1000, "Operation took #{duration}ms, expected < 1000ms"
+```
+
+```elixir
+# Example: Query count assertion (using Ecto query logging)
+# Verify no N+1 queries by checking query count
+```
+
+---
+
+## Monitoring and Maintenance
+
+### Regular Review
+
+- **Quarterly Review:** Review slow tests quarterly to ensure they're still relevant
+- **Performance Baselines:** Update performance assertions if legitimate performance improvements occur
+- **Test Cleanup:** Remove or optimize tests that become redundant
+
+### Success Metrics
+
+- ✅ Performance tests catch regressions before production
+- ✅ Standard test runs complete in < 3 minutes
+- ✅ Nightly builds complete successfully
+- ✅ No false positives in performance tests
+
+### Troubleshooting
+
+**If a performance test fails:**
+
+1. **Check if it's a real regression:** Compare with previous runs
+2. **Check CI environment:** Ensure CI has adequate resources
+3. **Review test data:** Ensure test data setup is correct
+4. **Check for flakiness:** Run test multiple times to verify consistency
+
+**If a performance test is too slow:**
+
+1. **Review test implementation:** Look for inefficiencies in test setup
+2. **Consider reducing dataset size:** If still representative
+3. **Split into smaller tests:** If testing multiple concerns
+
+---
+
+## Related Documentation
+
+- **Test Optimization Summary:** `docs/test-optimization-summary.md`
+- **Seeds Test Optimization:** `docs/test-optimization-seeds.md`
+- **Testing Standards:** `CODE_GUIDELINES.md` - Section 4 (Testing Standards)
+- **CI/CD Configuration:** `.drone.yml`
+
+---
+
+## Changelog
+
+### 2026-01-28: Initial Setup
+- Marked 5 performance test suites with `@tag :slow` or `@moduletag :slow`
+- Added `test-fast`, `test-slow`, and `test-all` commands to Justfile
+- Updated CI to exclude slow tests from standard runs
+- Added nightly CI pipeline for slow tests
+- Created this documentation
diff --git a/test/membership/group_integration_test.exs b/test/membership/group_integration_test.exs
index 2333a66..8e36119 100644
--- a/test/membership/group_integration_test.exs
+++ b/test/membership/group_integration_test.exs
@@ -79,6 +79,7 @@ defmodule Mv.Membership.GroupIntegrationTest do
     end
   end
 
+  @moduletag :slow
   describe "Query Performance" do
     test "preloading groups with members avoids N+1 queries", %{actor: actor} do
       # Create test data
diff --git a/test/mv_web/live/group_live/index_test.exs b/test/mv_web/live/group_live/index_test.exs
index 7dabcab..e844bcc 100644
--- a/test/mv_web/live/group_live/index_test.exs
+++ b/test/mv_web/live/group_live/index_test.exs
@@ -114,6 +114,7 @@ defmodule MvWeb.GroupLive.IndexTest do
     end
   end
 
+  @moduletag :slow
   describe "performance" do
     test "page loads efficiently with many groups", %{conn: conn} do
       # Create multiple groups
diff --git a/test/mv_web/live/group_live/show_test.exs b/test/mv_web/live/group_live/show_test.exs
index af298fd..177318e 100644
--- a/test/mv_web/live/group_live/show_test.exs
+++ b/test/mv_web/live/group_live/show_test.exs
@@ -219,6 +219,7 @@ defmodule MvWeb.GroupLive.ShowTest do
     end
   end
 
+  @moduletag :slow
   describe "performance" do
     test "member list is loaded efficiently (no N+1 queries)", %{conn: conn} do
       group = Fixtures.group_fixture()
diff --git a/test/mv_web/member_live/index_membership_fee_status_test.exs b/test/mv_web/member_live/index_membership_fee_status_test.exs
index 043c5cb..f868e96 100644
--- a/test/mv_web/member_live/index_membership_fee_status_test.exs
+++ b/test/mv_web/member_live/index_membership_fee_status_test.exs
@@ -238,6 +238,7 @@ defmodule MvWeb.MemberLive.IndexMembershipFeeStatusTest do
     end
   end
 
+  @moduletag :slow
   describe "performance" do
     test "loads cycles efficiently without N+1 queries", %{conn: conn} do
       fee_type = create_fee_type(%{interval: :yearly})
diff --git a/test/mv_web/member_live/index_test.exs b/test/mv_web/member_live/index_test.exs
index 0624c77..388f70d 100644
--- a/test/mv_web/member_live/index_test.exs
+++ b/test/mv_web/member_live/index_test.exs
@@ -1775,6 +1775,7 @@ defmodule MvWeb.MemberLive.IndexTest do
       assert Enum.any?(boolean_fields_after, &(&1.name == "Newly Added Field"))
     end
 
+    @tag :slow
     test "boolean filter performance with 150 members", %{conn: conn} do
       system_actor = Mv.Helpers.SystemActor.get_system_actor()
       conn = conn_with_oidc_user(conn)