As XCthulhu mentions, early versions of OS/X were less than usable on simultaneously available G3 processors. So when one of my friends decided he wanted a laptop, he was heisitant to blindly run out and buy an iBook. Now you'd think that if you were very specific about processor speed and OS version, you could get an impression in forums and Usenet what kind of snappiness to expect, however it turns out that GUI performance is both poorly-defined and ineffable. So I figured if I used my secret Computer Science training for good, I could tell him what causes desirable qualia and how to test for it.

GUI Performance is primarily dependent on three things:

  1. The operating system's ability to preemptively multitask. It's much more noticable if the pointer movement takes an extra half-second than if your thirty minute render does (in VR lag makes you throw up); so it's important that the operating system prioritise the GUI and give it resources as soon as it needs them. Many real-time systems have GUI-response requirements.
  2. The operating system's ability to manage virtual memory. Swapping both increases overhead and makes multitasking inefficient, so this ties in to the other two criteria. In my experience, this is a frequent cause for bad GUI performance in Windows 9x systems.
  3. The overhead involved in user interface interaction. Windows 9x, OS9, OS/X, Windows NT, and X Windows* all have significantly different windowing architectures. As a result, completely different things are going on behind the scenes when you click and stuff.

Q: How can you check how an OS handles these three things?

A: Run something in the background that takes up all the clock cycles and physical memory and then perform stupid GUI tricks: dragging, zooming, transparent windows, etc. This should give you the best idea of worst-case performance.

I suggest bringing a CD to a store with an MPEG to play at fullscreen, a Photoshop filter to run on a large file, or raw video to render.

* This is in rough order of fastest to slowest: the insecure OSes can access the hardware more directly while X windows's client-server architecture trades flexibility for performance. I'm not an expert on any of these windowing systems, so I may be wrong. If you are an expert or look into it further and I'll hardlink.