Window resizing is extremely slow and consumes a lot of CPU. Half by Qt application itself, half by X11.
It's not a Qt regression since this bug exists for a long time (starting Qt 5). Qt 4 worked fine, but I don't have one at the moment to test again.
The resize lags only in Qt applications, not GTK+, so it's not KDE, X, nvidia bug. Event through that X11 consumes a lot of CPU too and according to profiling it's because of nvidia. GTK+ windows in GNOME works perfectly smooth.
More details: https://bugs.freedesktop.org/show_bug.cgi?id=103362