Apart from your valuable information, as far as I understand it, there is some work in progress regarding Krita being slow in various circumstance which can’t be solved with more powerful hardware.
There seems to be something within Krita that let it struggle - not with painting but with using filters etc.
You can read about it here:
The post was initially only about OpenGL vs. Direct3D acceleration but gathered more input - e.g. Krita getting slow as soon as you add a blur filter.
Following the forum for some time I got the conclusion that Krita’s brush system is considered fast, even on old slow PCs (and I can confirm this on a ten years old notebook). But “editing” a document (using filters, layer styles etc.) can create a performance bottle neck.