Response time. The time between clicking the mouse and seeing a change on screen feels instantaneous; there's just no hesitation or lag for the most part. Compute-intensive tasks can't compete of course, but response time has gotten much worse in modern operating systems, especially mobile ones. It's quite common on a modern Mac for a scroll of a Finder window to put up a spinning beachball. Wristwatch cursors happened on classic Macs too, but never for something as trivial as a scroll. Part of the problem is that we're still using O[n] algorithms where n is a lot bigger than it used to be, but another part is that we no longer write UI code with an emphasis on quick response time.