Well. So far I got no answers in the Krita forum, btw.
What I did today: On a Lubuntu system on an SD card (because Nvidia prime is functional there and I can switch between Intel and Nvidia), I made some performance tests, though not extensive performance tests. Apparently there is no or very little difference between Intel and Nvidia. However, I think Krita does not use OpenCL but something else. Something does use GPU acceleration in Krita. Maybe its some brush or something like that, not filters. But I think I need to do more tests. Apparently there is very little official documentation about Krita and GPU acceleration.
Edit: Actually and SSD appears to have more impact on speed in Krita (than Intel or Nvdia GPU).