My personal experience in using QT apps is that they load / render perceptibly slower than the native UIs and QT applications are slightly bulkier in size (storage space and memory). This is kind of understandable as QT emulates the UI through its own library (unlike, say, Lazarus IDE / Delphi GUI library that directly use the native UI library). However, size doesn't matter much today, and the very minor rendering d…
What operating system are you referring to?
On Linux, Qt does have any underlying UI to emulate. On many others, my understanding is that this "emulation" is just theming. And on others it requires to draw on non optimized video buffers, which then become noticeable.