When you code on a GUI (OS/2's GUI, Windows, etc...) you don't know what the low-level manager does. And you don't care about it as this isthe manager that does it :-) For example, when you move a window, it must test its position in the stack of the visible windows, mask the other windows when it wants to display something. The most complicated being the clipping process. Not only you have to clip with screen borders but with other windows too !!! You cannot put back the content of your windows so simply ! Here is an example:
Here, to display the A window contents, you'll have to clip it with all of the other windows ! That's a real pain...and the main reason for windows display slowness !