One of the major differences between X Window and the win32 GUI APIs is that the windows one builds in thread safety, and it cannot be removed. This means that you pay the price of mutexes and the like (what the windows world likes to call "critical sections"), even if you have a single threaded GUI. X Window, on the other hand, decided to do nothing about threads at all, leaving it up to the application.

30 years after these decisions were made, most sensible people do single threaded GUIs anyway (that is, all calls to the windowing API come from a single thread, and all redraws occur synchronously with respect to that thread; this does not block the use of threads functioning as workers on behalf of the GUI, but they are not allowed to make windowing API calls themselves).

Consequently, the overhead present in the win32 API is basically just dead-weight, there to make sure that "things are safe by default".

There's a design lesson here for everyone, though precisely what it is will likely still be argued about.