How do I increase the number of actions per batch?

Version: 4.1.14.3

I have a board that got hammered by a spam bot, leaving me with nearly 900k spam posts. I'm trying to delete the users and clean the posts, but it's running at 50 per cycle. At this rate, it will take me a week to complete.

I have the site running locally while I clean up the database, so I know I can handle way, way more actions at a time. Where can I adjust the number of actions per batch?

Any help would be extremely appreciated.