This problem happens on a rather old machine (athlon 64 6400+x2, 4GB memory, AMD radeon 4670 HD [but still appeared with a NVidia 630 GT]), on which brush movement is still usually perfectly smooth.

When OpenGL is activated and canvas is rotated, at some high zoom range (e.g 1000% to 1500% for a 4096x4096 canvas, though these values seems to vary), the brush become laggy and its position update can become as slow as two or three per seconds, except is rotation value is /near/ 0, 90, 180 or 270 (±5°) , in which case no slowdown is visible. Sometimes, the problem also disappears at very high zoom values (2000+) and is never present at all for small zoom values (<1000).

Using texture buffer or not, changing interpolation mode has no influence.

However, this never happens when OpenGL is disabled, no lag is to report. However, it makes zooming and rotating so slow that it doesn't represent the best workaround.

Reproducible: Always

Steps to Reproduce:
1. Open Krita x64, create a 4096x4096 canvas (tested in 8-bit grayscale, with a white canvas color)
2. Be sure to use OpenGL for canvas rendering
3. Do some scribbling to see the canvas zoom/rotations with the classical pen tip brush.
4. Rotate a bit (~30°), zoom in slowly, while being attentive to the fluidity.
5. When lag starts to become noticeable, stop zooming and try to draw some lines, the brush should be laggy.
5. Rotate near 0° while keeping the same zoom value, check the brush fluidity again, it should be perfectly responsive now.


Actual Results:  
Near 1000+, the canvas starts to lag during zoom itself , and it will lag more and more until reaching a certain zoom value and instantly become responsive again.

Right before this value, the canvas will lag the most, and brush is near unusable, until the canvas is rotated to an almost horizontal/vertical angle, in which  case lag disappears entirely.

Expected Results:  
Well, being responsive for every zoom/rotation values, since the lag phenomenon isn't always present, only when certain conditions are met.

The configuration : 
Athlon 6400+x2
AMD radeon 4670 HD (formerly a NVidia 630 GT)
4GB memory
Running Windows 7 Professionnal Edition 64 bits
Using a Wacom Intuos Creative Pen & Touch and Krita 2.9.10 (labelled 2.9.9 on splash screen) 64 bits