yes, but after upgrading to 2.9.1 the problem continued. I took a look at the changelog and saw new feature "Parallel optimization". I now deactivated this option to see if that resolves the problem.
But it's still strange because the database is in an isolated server (Amazon RDS) and the plugin uses the cloud to conversion.
The site environment (nginx + php-fpm + memcached) has affected the performance when this problem occurs in the database.