DirectX:
Made a screen cap of the problem here . I'm running on Windows 7 64bit.
Painting with any brush on that top layer makes those weird light colored boxes around the area where I paint. Turning off the texture buffer doesn't make a difference, nor does changing the scaling mode. But it seems to work OK using openGL so far though.

View > Instant Preview Mode:
Also, the cursor seems to "stick" or hang there after selecting a color then painting with the brush. I tend to paint quickly, holding the alt key to select a color, then releasing it to paint, so this lag really starts to become annoying after 20 or so of these "quick dabs". Turning the instant preview mode off seems to fix this though. The preview might be "instant" but the lag shows up immediately after trying to paint again. Don't really understand the usefulness of this feature

Also, I apologize if these seem like common or 'nagging' complaints. I use PaintToolSAI v2 on a daily basis and tend to compare every painting program to it in terms of speed, especially with bigger brushes. I would love to see Krita become that fast with painting

I'm not sure what "light coloured boxed you are referring to. Can you record a video showing the problem appearing in real time?

I was talking about the odd stair-step pattern that I outlined in red here: https://imgur.com/JMjZr98

Can't really seem to reproduce it anymore using the same settings, otherwise I would. It felt like a bug with the instant preview mode or the way the strokes are rendered.