In the Fast mode only the picture being worked on is redrawn, which happens very quickly since other frames on the page can be ignored. This can mean though that the contents of other frames which may lie on top of the processed picture might simply appear to be Painted over on the screen. But don't worry: This is only a temporary effect - the contents of the frame are not affected in any way, and are restored when either the page is redrawn, another frame is selected or the Paint module is quit.
