Thanks for the thoughtful analysis and napkin math. You may very well be right. I wonder if this is true in practice or if they suffer from any interface abstractions and whatnot.
On every modern (past few decades) platform, the mouse cursor is a hardware sprite with a dedicated, optimized, *fast* path thru every layer of the stack, just to shave off a few ms of user-perceived latency. Grab a window and shake it violently, you'll notice it lags a few pixels behind the cursor - that's the magic in action.
In some places there's no room left for unnecessary abstractions, I can imagine most of the code touching mouse / cursor handling is in that category.
