darktable on the other hand has basically one dark theme only which also shows stupid UI mistakes as black-overscroll-on-black-background in all lists and fixed font size. If there is ONE setting you have to keep, is the FONT SIZE in the system theme!
You're excluding many more issues from here. Click on a scollbar and drag outside: very often the scrollbar doesn't release when the mouse button is depressed, though styling and custom events play a role. QML is not immune to a lot of quirks in highdpi, most of which are completely absent in QtWidgets. In detail, font rendering (hinting) in QML in GL contextes is basically broken.
The whole UI is perceptibly laggier as a whole. I'm a programmer myself. As a programmer I feel ambivalent, but as an user, I hate it.
As far as slow, no way. Instant startup, instant everything, no memory leaks. I've left my editor running for multiple weeks straight with no issues.
Usability bugs? Yes, there's basically one: scrolling. I've hacked together my own momentum scrolling system but I'm not happy with it. At least it's better than scrolling in KDE Plasma native applications, though.