I've been using Krita over the last few days. I've not looked at it as a program for about 13 years. It has improved somewhat.
It's been a revelation actually. Much, much better than GIMP.

However - I've been using it on Archman Gnome and Ubuntu. After a period of time (I lose track of time whilst working), the machine freezes and has to be forcibly rebooted. This has happened repeatedly, on both OS's. Krita seems to save all open work, so that's not an issue.
My computer has 4GB of RAM - this is not currently something I can change - and I use the usual 4GB swap file in setup. So the machine seems to be running out of usable space? - my home folder had a total of 23GB of 'hidden' .kra files from the crashes. The only alternative I can think of is to wipe the drive (it's an SSD) and set it back up with a 20G+ swap file.
Is that likely to sort the issue?
I haven't yet looked into whether it's possible to assign a scratch file type thing like Photoshop. I have a 60GB SSD that could serve as such.

Those .kra~ backup files can really take up space if you get lots of them so they need to be pruned now and then.
Krita uses the /tmp folder for its swap file location (unless you've changed it) and that is its 'personal' swap file (named KRITA_SWAP_FILE_{random string)} and is nothing to do with the OS swap file/partition. That krita swap file does get left behind in a crash so you may have some pruning to do there as well.

You don't need to wipe the drive, just delete the .kra~ files and any orphaned swap files. Provided your /tmp partition has at least 4GB free then that should be enough.

With a fresh new image, I see krita using 1.1 GB and a total RAM usage of 1.9GB (Debian 9) That leaves about 2GB available on a 4GB RAM system which can get eaten up and cause krita to go swappy. Did your working krita image start to go towards 2GB as noted on the status bar at the bottom?

Which version of krita are you using and where did you get it from? Have you tried using the appimage build? If not, I suggest that you do from https://krita.org/en/download/krita-desktop/ and maybe even the latest 4.2.0-beta from https://binary-factory.kde.org/job/Krit ... age_Build/ to see if it gives you a smoother experience.