High load when moving/deleting a bunch of mails

[glotzkowsky]

Hi,
I have a Kopano server on Debian 9. It’s a VM on non SSD hardware (should be way enough to handle some mails) and five users. So the system isn’t really under load. It has two CPUs and 6GB RAM (doesn’t swap)
But when a user deletes/moves a bunch of mails (about > 50) the Mysql server produces high IO load and the system is really slow sometimes unresponsive until the deletion is finished. So other users are affected.
Is there a way to optimize that? What about servers that serve > 1000 Users?
Of course these times SSDs are the solution for all IO problems. But I think optimizing should be the right way.

Regards
Markus