Proper way to snap widget to mouse and ensure fluent movement.

frogi16

I want to allow users to modify position of widgets on screen. It would be nice to have some kind of preview how it looks so currently in mouseMoveEvent window calls move function of the suitable widget. This solution has some flaws:

Movement seems delayed when mouse moves quickly.
My test object (green rectangle) looks like it was getting smaller when it moves. Probably some drawing error which I can't really understand. I suspect that may have something with refresh rate because when I try to print screen and document this behaviour I see rectangle with normal size.

So, what approach should I take? Thanks for the answers.