Every weak a new curiosity...
If you go to maximum zoom, 6400% , anywhere on canvas, and rotate, there is no slowdown in any angle, until you come to specific zoom... to me it was 1131%, windragon stated 1600%. It was tested with git b22665a.

I've found position of canvas affect the range of problematic angles, with canvas precisely centered with help of zoom, it works fine in angles as 45º. In fact the problem resumes to a small range near 8º. If I did not change the zoom, canvas stay normal, even if I goo away from canvas center, no slowdown, but the moment I change zoom, the problem returns.

I've updated my drive to 361.28 and it still behave the same.
I did more tests and lag disappear with OpenGL off, at last the increased effect by zoom, but brush still is not fluid moving as it is with 2.9.x and OpenGL on.

If it is of any help:

I can confirm it isn't a case of OpenGL off like second report , here it is on, and the problem affects all operations in canvas on non orthogonal angles rotation, like anything between 0º and 90º.

I've noticed that angles like 0º, 90º, 180º... did not suffer from slowdown on those increased zoom levels. The nearest you go, the slower it gets, nearly unusable on 3000%+. Here scaling modes did not made difference.
2.9.9 LOD testing from ppa:dimula73/krita did not suffer from this too.
I can confirm it affects both my NVIDIA 525m and Intel second generation video card, on both Kubuntu 15.10 and Ubuntu 16.04