If your device is still very slow (meaning something is really wrong since Leste on N900 is quite fast now when updates are ok), i doubt overclock could help at the moment.
I think you should wait to get a clean and fast install before running Mypaint and overclock.

BTW i tried to install Mypaint on my device, it took around 2 minutes but something is wrong with icons and i can't run the app. Any tips to get it working ?