Well, I’m starting to think this is some kind of hardware problem.
Strokes on left are in Win 10, on the right, Mint 20. There does seem to be some of this unevenness on the Windows ones, too, although much less. Unfortunately the strokes have that other problem I have with the Art pen on Windows; blobs at the start.
The pen is unlikely to be broken, because I tried with a different Art Pen and it is the same.
I also tried, after having quite a time getting all these to boot, live USBs of mint 17, 18 and 19, and the same problem persists there.
I also tried connecting the Cintiq to a laptop running mint 18, same problem.
Edit: It is the pen, somehow. The eraser does not have this problem! And, testing with evtest, I get about 0.7-0.9 ms refresh rates for the pressure signal with the eraser, so about 10 milliseconds less.
Gahh! What now? I really do not want to not use the art pen, the rotation is pretty important to me. Do I need to get in touch with Wacom about this? Or the linuxwacom devs?
Feel pretty screwed still, because I can not work until this is fixed somehow.
Edit2: I’m not sure what complaining about this here is supposed to help, but anyway: running evtest, I can see that data for pressure, tilt etc. is only returned every other time the pen is polled. So that would explain the low refresh rate.