The default renderer setting for Krita 4.2.8 is "Auto (OpenGL)". It stutters while drawing with a brush, in a frame-skipping sense, whether by mouse or drawing tablet. Doesn't matter whether a big or small canvas. I have Aero turned off (W7 classic theme), have not tested with it turned on.

Direct3D 11 ANGLE: Perfectly smooth drawing, presumably a constant 60 FPS
Software Renderer: Delayed drawing, but still smooth, feels like a constant 25 FPS or something, and therefore still usable
OpenGL: Average FPS is probably 50-59 FPS, but the microstuttering (frame-skipping) every half-second or so while drawing is so distracting I'd rather settle with the lower-FPS-but-consistent Software Renderer if D3D11-ANGLE didn't exist

I wonder what's wrong with the OpenGL implementation to cause the microstuttering.

Software: Krita 4.2.8 (64-bit); GPU: GTX 960 2GB; GPU driver version: 388.84; Tablet: Huion 580; OS: Win7 Pro SP1

Nothing; the problem is with the GPU driver. On Windows, the OpenGL drivers for a given GPU are often much worse than the direct3d drivers. Krita uses OpenGL, and on Windows that can be translated through Angle to d3d, or use the OpenGL driver directly.

Can you set it so the next version's Auto option is ANGLE D3D11 instead of OpenGL for Windows 7 and newer (iff the GPU is detected to support the required feature level)?
It is fixed by gpu driver update.