Hi, this is a task discussed on #kritasprint2016.
The problem: Sometime Krita use more RAM than available (big animation, many document opened) and start to swap memory and become slow.
Actual solution Save all, and restart Krita.
Proposal: Let the user to hit a button to 'flush the undo history' and free a bit of the RAM available.

Little mockup idea, in two steps:

1. click on the bottom bar 'memory tag usage', a new button is available on the tooltip to flush the undo history
2. a confirmation dialog appear to warn the user.