After an Update to 4.2 archiving has become extremly slow, even when using auto-archive with a cronjob:
/usr/bin/php /path/to/matomo/console core:archive --url=http://example.org/matomo/
mytop shows that temporary tables are created and dropped very slowly and the archiving does produce high server load. All this came with the update to 4.2 and was not a problem before.
On that server Matomo is running on PHP 7.3 and MariaDB 10.0.38.