I'm running Krita on x64 Arch, all the drivers are in place, but I'm getting some lag.
Well, not exactly lag.
Every couple of minutes during drawing the whole window kind of freezes for a few seconds and I can see the bottom panel being re-drawn, then everything goes back to normal until it happens again.
It used to be much worse until I enabled the "use texture buffer" option.

I have a GTX 650 (MSI) with 1GB of memory and 5GB RAM, the picture is 300 ppi A4 and I was using some very basic circle brush.

Can you check if it still happens when you disable autosaving?