If one create multiturn qwtKnob with total tick count more than 1000 it significantly slows down application during qt layout calculations and first painting.
The problems are in QwtRoundScaleDraw class in functions extent() and draw(). The second one is from QwtAbstarctScaleDraw. Extent() use all more 1000 ticks for estimation, but it realy needs only tick which is related to first turn. The problem with QwtAbstarctScaleDraw is the same, it tries to draw all ticks, but in reality it draws only ticks related to the first turn.
I have made some workaround for this problem. In extent() break was added, if angle for tick is more than 360. Draw() was reloaded in QwtAbstarctScaleDraw and breake was added for same condition. Patch is attached.