It's an oversimplification to say the the four corners of the screen are the fastest targets to point at. According to Fitts' Law, the time to point to a target

It's an oversimplification to say the the four corners of the screen are the fastest targets to point at. According to Fitts' Law, the time to point to a target is a function of the distance to the target and the size of the target. In some cases that may be the corners of the screen (because they effectively have an infinite target size). But if the corners are very far away (as can be the case on large screens), then other nearby targets can be faster to acquire. So, on a small screen, the Mac approach might be faster. On a large screen where the window size is a smaller fraction of the display size, the Windows approach might be faster. But Fitts' law doesn't tell the whole story -- there's more to consider than just the raw target acquisition speed. What about the time you spend figuring out where the target is? That's probably lower with a single menu bar, because it's always in the same place no matter where the window is. But if the application has lots of controls in the toolbar, you might find that people instinctively start moving towards the toolbar first, before realizing that the command they need is actually in the menu bar. On Windows, it's a slight correction to start heading for the menu bar; on OS X, it might be a completely new movement. A better example of how the multiple menu bar approach might be better: on a large monitor, when I have several non-overlapping windows, I sometimes head to the menu bar, only to realize that the wrong application is active. So I have to move the mouse again, select the correct window, then go back to the menu bar to make my selection. With multiple menu bars, this problem doesn't occur.