I'm having an issue where I'm not sure if this is considered lag or not. I will draw a line and it will show up on the layer preview on the side, but not the actual picture. The cursor won't move either and it will stay like that at one point on the canvas. If I want to see the line on the canvas or see m cursor in a different spot, I have to click on an application outside of Krita and click back in. Same goes for if I want to erase, undo, or type something. I can move my cursor on the color area, the tools, layers, etc. But the canvas is just not cooperating. I'm not sure what to do as it was working fine a couple days ago.
For those wondering, I have Krita version 3.1.2 and windows 10

And you have an Intel GPU, I guess, and a broken driver...

This is the Number One issue we're seeing these days: it seems a recent Windows 10 update just broken EVERYTHING for EVERYONE. I'm in touch with Intel to see what's up; you can help by telling me exactly what GPU and display driver you have, so I can pass that on to Intel. In the meantime, you can go to settings/configure krita/display and disable opengl and continue painting.

My GPU and Driver, at least according to Windows Diagnostic, is Intel(R) HD graphics card and my driver model is a WDDM 1.3. Hope this helps. And disabling the opengl certainly did help, thanks for that.