I do understand the technical reasons behind it, and it has nothing to do with bloat. It mostly has to do with mapping a cross-platform (i.e. generic) window abstraction onto a specific OS-provided drawing/event/windowing API.
I built my own cross-platform windowing/event API from scratch at my last job, based on top of win32 on Windows, Cocoa on Mac and Qt on Linux. The only platform where there was any noticeable performance drops at all was Linux, because it relied on Qt.
When I said "I don't understand the technical reasons behind it", I didn't mean "I don't understand how to abstract OS-native APIs". I meant "I don't understand how a team of intelligent people managed to fuck things up this badly".
There are some reports of bad things happening with inefficient QPainter usage, and some bugs related to text drawing especially after qt5, after a cursory look...would be better to provide specific examples. I do rather suspect something machine or even program specific, though.
