diff --git a/.gitlab-ci/pipeline.yml b/.gitlab-ci/pipeline.yml
index e766c651ca58fc8726467851ac4bdb7bf1a8f708..6cda6e2f978d98b1c07bb751b825d3a4a0c59de3 100644
--- a/.gitlab-ci/pipeline.yml
+++ b/.gitlab-ci/pipeline.yml
@@ -160,6 +160,7 @@ variables:
   variables:
     TESTSUITE: PHPUnit-Build
     KUBERNETES_CPU_REQUEST: "2"
+    CONCURRENCY: "4"
   services:
     - <<: *with-database
 
diff --git a/core/tests/Drupal/BuildTests/Command/GenerateThemeTest.php b/core/tests/Drupal/BuildTests/Command/GenerateThemeTest.php
index e4f7c40f2005e4f5a63a945d37a1411aeff24a5c..3a2eeb13d5517f2460d051dc26061fa1b8658057 100644
--- a/core/tests/Drupal/BuildTests/Command/GenerateThemeTest.php
+++ b/core/tests/Drupal/BuildTests/Command/GenerateThemeTest.php
@@ -19,6 +19,7 @@
  * @requires extension pdo_sqlite
  *
  * @group Command
+ * @group #slow
  */
 class GenerateThemeTest extends QuickStartTestBase {