I found that the primitives for threading and message passing between threads didn't work that well for me (both in terms of what they provide and in terms of performance) so I ended up "bypassing" them a bit with my own threading code (that is, still using QThread and QMutex etc. but not using the signals and slots model).

Once I've done that and moved everything off of the main thread, things worked well, so that program is still in QtWidgets with C++ and still in use, but I felt that Qt didn't do a good job of making multithreading easy for me.

That said, I still like continue to use Qt so you can consider this a sort of mild endorsement from me, I guess.