QImage pixel manipulation access time/performance question

[high_flyer]

I use this code to show "preview" and I can see, while changing slider (connected to the value)

It also can be that you are running the slot for each value change which will restart your loop again and again.
Try to build in some logic either time based on mouse release, to only run the preview loop when its more or less certain that the current slider value is the value that needs to be previewed.