The UI is totally performant for me. The only time it's bogged down was when I was modelling something with > 100k mesh triangles. Or when I do a long UI-blocking processing operation (which isn't in the qt codepath)./
What I'm trying to understand: are you talking about when you ask Fusion 360 to do something expensive the UI stops responding for a bit? I certainly see that, but it's not Qt that is the source of problems.
Do you work with huge models?
For hobbyist stuff at least Fusion is by far the least painful option, but the bar is set really, really low. FreeCAD is a gigantic clusterfuck to put it charitably (akin to using an awl to carve a drawing out of cardboard versus pencil and paper). OpenSCAD is neat but it really suffers because OpenSCAD is basically developed and maintained by a single person.