I just upgraded my 2008 Macbook Pro (the one with the notorious NVidia 8600M GT that I've had no trouble with here) to Mountain Lion last night and after getting everything reorganized and updated (rid/replaced PPC stuff, etc.), it just seemed like the interface felt slower. Well, ML has more eye candy and I just got a 2012 quad-core Mac Mini a few months ago and it feels like lightning so perhaps it was my imagination?
I realize XBench is ancient but it's still running the same tests between OS versions and identical OS machines to give a reasonable estimate of differences over several passes.
I had to disable the "thread" test (now freezes in ML), but it was interesting to see the comparisons. I even compared regular Leopard results I had saved as well (10.5.6 version), although that was before my 2GB -> 4GB ram upgrade and the internal drive from its stock 5200 RPM one to a 500GB 7200 RPM Seagate.
But here are the main Differences in order from 10.5.6 to 10.6.8 to 10.8.2 on the 2008 Macbook Pro (rounded):
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CPU
10.5.6: 162
10.6.8: 170
10.8.2: 172
Mountain Lion wins for efficient CPU usage and OSX has improved steadily in this test.
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Memory
10.5.6: 162
10.6.8: 174
10.8.2: 184
Again, Mountain Lion appears to use memory more efficiently, although the Leopard test had 2GB of Apple Ram whereas SL and ML had 3rd party 4GB ram.
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QUARTZ (2D Graphics)
10.5.6: 220
10.6.8: 208
10.8.2: 270
Once Again, Mountain Lion appears to show OSX improving over time on the same hardware.
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OpenGL (3D rendering)
10.5.6: 160
10.6.8: 157
10.8.2: 96
Here, we see continual drops in performance on the same hardware over time with a MASSIVE drop in Mountain Lion compared to either Leopard or Snow Leopard. I ran this test a few times and it varied somewhat, but ML just plain did horrible. Of course newer hardware would do better, but this is apples to apples using an older test software as well. Most older games aren't going to change so it seems valid to me.
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User Interface
10.5.6: 340
10.6.8: 295
10.8.2: 245
Once again, we see a continual drop in performance. This is real noticeable performance as well. I noticed a definite slowdown in the feel of Snow Leopard over Leopard and it's pretty clear that ML's eye-candy handling of windows and screens feels slower than Snow Leopard as well (although some of it's new so it's not as easy to compare directly). I'm not sure what's going on here and if it relates to all OpenGL stuff, but that's a pretty sad score given the same hardware.
My Mac Mini has scores for OpenGL that are 2.5x higher, User Interface is 1.8x higher and Quartz is also 2x higher and that's with an Intel 4000 so clearly even Intel's integrated chips are over 2x as fast as the 8600M GT (which wasn't bad for its day for a mobile GPU).
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Disk Test
10.5.6: 39
10.6.8: 48
10.8.2: 56
The hard drive is different under 10.5.6 (5200 RPM Apple vs 7200 RPM Segate) so you'd expect the 10.6.8 score to improve, but 10.8.2 is clearly faster yet for getting the maximum out of the hard drive. I got 110MB/sec writes vs. a mere 80MB/sec in Snow Leopard with the same drive.
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Overall, Mountain Lion appears to be genuine improvement in all the tests over Snow Leopard except OpenGL and User Interface. Unfortunately, these are two HUGELY important areas for a "snappy feel" to the interface and window drawing behavior along with potentially (depending on how it behaves with real world games) affecting OpenGL 3D games quite a bit to the negative on the same hardware. I'll have to try some games to be sure, though.
Leopard was mostly a drop compared to Tiger as well on my PowerMac PPC machine in similar areas. In short, the interface/window/3D behavior appears to becoming BLOATED and relying instead of ever newer/faster GPUs to make up for it. Sadly, in some cases the newer GPUs are SLOWER than the last generation (e.g. Mac Mini's GPU on the model with the Radeon).