Your code is subscribing to a kernel event dispatched on every request.

Try 

    static function getSubscribedEvents() {
        $events['mymodule_user_login'][] = ['onUserLoginCallback', 1];
        return $events;
    }

BTW the dispatching code is deprecated. In Drupal 9 or higher swap both arguments:

    $event_dispatcher->dispatch($event, 'mymodule_user_login');