[renox]

I'm not it's a Qt issue: it could be a lack of hardware acceleration on Linux's GPU driver..

[AussieWog93]

This is something I've observed cross-platform (Windows, macOS and Linux) across both nVidia and AMD machines with updated drivers. Actually try it.

Get your favourite Qt program, resize the window like a madman and watch your CPU usage launch into the stratosphere.

[smaudet]

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.