diff --git a/core/scripts/run-tests.sh b/core/scripts/run-tests.sh
index 64ae223..8226271 100755
--- a/core/scripts/run-tests.sh
+++ b/core/scripts/run-tests.sh
@@ -457,6 +457,7 @@ function simpletest_script_init() {
   $_SERVER['HTTP_USER_AGENT'] = 'Drupal command line';
 
   if ($args['concurrency'] > 1) {
+    $args['concurrency'] = 10;
     $directory = FileSystem::getOsTemporaryDirectory();
     $test_symlink = @symlink(__FILE__, $directory . '/test_symlink');
     if (!$test_symlink) {