It can happen anytime, but doesn't always happen. A pretty reliable way to reproduce it for me is to drag a window around in large circles on the screen at a rate of about one circle per second. When I do that, the window lags quite a ways behind the mouse cursor. Curiously, I haven't noticed any lag when doing other things, such as dragging scrollbars or playing games.
It can happen anytime, but doesn't always happen. A pretty reliable way to reproduce it for me is to drag a window around in large circles on the screen at a rate of about one circle per second. When I do that, the window lags quite a ways behind the mouse cursor. Curiously, I haven't noticed any lag when doing other things, such as dragging scrollbars or playing games.