I know Eric said there is no such option on https://www.virtualmin.com/node/25989, but it was two years ago, so maybe we can think this through and implement as a new feature.
The problem is that Drupal websites that we host always have huge cache tables, which make backups very slow and often cause errors. If only Virtualmin backups could give end users a possbility to exclude huge cache tables in the website's databases, then it would be so much relief to lot's of Virtualmin servers.
I understand that when backing up multiple domains with different database structures this could be a challenging task. On the other hand it seems so easy to feed an exclusion filed in form "wildcard-anyword-wildcard" to the backup script to address this issue.
EDIT: At least running OPTIMIZE for each database table prior backing up would help to improve performance of all Virtualmin servers.
What do you think, Jamie?