diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index 434f71ddfa..1fc40c8332 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -24,6 +24,7 @@ env:
   # Controls which NPM script to use for running PHPUnit tests. Options ar `php` and `php-composer`.
   PHPUNIT_SCRIPT: php
   LOCAL_PHP_MEMCACHED: ${{ false }}
+  SLOW_TESTS: 'external-http,media,restapi'
 
 jobs:
   # Sets up the workflow for testing.
@@ -66,7 +67,7 @@ jobs:
   # - Submit the test results to the WordPress.org host test results.
   # - todo: Configure Slack notifications for failing tests.
   test-php:
-    name: ${{ matrix.php }}${{ matrix.multisite && ' multisite' || '' }}${{ matrix.memcached && ' with memcached' || '' }} on ${{ matrix.os }}
+    name: ${{ matrix.php }}${{ matrix.multisite && ' multisite' || '' }}${{ matrix.split_slow && ' slow tests' || '' }} ${{ matrix.memcached && ' with memcached' || '' }} on ${{ matrix.os }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false
@@ -74,8 +75,20 @@ jobs:
         php: [ '5.6.20', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0' ]
         os: [ ubuntu-latest ]
         memcached: [ false ]
+        split_slow: [ false ]
         multisite: [ false, true ]
         include:
+          # Additional "slow" jobs for PHP 5.6.
+          - php: '5.6.20'
+            os: ubuntu-latest
+            memcached: false
+            multisite: false
+            split_slow: true
+          - php: '5.6.20'
+            os: ubuntu-latest
+            memcached: false
+            multisite: true
+            split_slow: true
           # Include jobs for PHP 7.4 with memcached.
           - php: '7.4'
             os: ubuntu-latest
@@ -189,22 +202,37 @@ jobs:
       - name: Install WordPress
         run: npm run env:install
 
+      - name: Run slow PHPUnit tests
+        if: ${{ matrix.split_slow }}
+        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ${{ env.SLOW_TESTS }}
+
+      - name: Run PHPUnit tests for single site excluding slow tests
+        if: ${{ matrix.php < '7.0' && ! matrix.split_slow && ! matrix.multisite }}
+        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --exclude-group ${{ env.SLOW_TESTS }},ajax,ms-files,ms-required
+
+      - name: Run PHPUnit tests for Multisite excluding slow tests
+        if: ${{ matrix.php < '7.0' && ! matrix.split_slow && matrix.multisite }}
+        run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --exclude-group ${{ env.SLOW_TESTS }},ajax,ms-files,ms-excluded,oembed-headers
+
       - name: Run PHPUnit tests
+        if: ${{ matrix.php >= '7.0' }}
         run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }}
 
       - name: Run AJAX tests
+        if: ${{ ! matrix.split_slow }}
         run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c ${{ env.PHPUNIT_CONFIG }} --group ajax
 
       - name: Run ms-files tests as a multisite install
-        if: ${{ matrix.multisite }}
+        if: ${{ matrix.multisite && ! matrix.split_slow }}
         run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c tests/phpunit/multisite.xml --group ms-files
 
       - name: Run external HTTP tests
-        if: ${{ ! matrix.multisite }}
+        if: ${{ ! matrix.multisite && ! matrix.split_slow }}
         run: npm run test:${{ env.PHPUNIT_SCRIPT }} -- --verbose -c phpunit.xml.dist --group external-http
 
       # __fakegroup__ is excluded to force PHPUnit to ignore the <exclude> settings in phpunit.xml.dist.
       - name: Run (xDebug) tests
+        if: ${{ ! matrix.split_slow }}
         run: LOCAL_PHP_XDEBUG=true npm run test:${{ env.PHPUNIT_SCRIPT }} -- -v --group xdebug --exclude-group __fakegroup__
 
       - name: Checkout the WordPress Test Reporter