Benchmarks are running tests on GPU, CPU etc...I'm running Mavericks 10.9.5 alongside El Capitan beta 6 on the same Mac Mini (late 2009) in the partitioned HD.
I did XBench on both the OSes and I got better results for Mavericks, far better specially in Quartz Graphics Test ( 209.36 vs 102.02 !? ) and User Interface Test (188.98 vs 18.71 !?!?!? ) , still El Capitan running faster and smoother than Mavericks in every aspects.
How it is possible?
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The OS response is dictated by the animation speed, fluidity - overall the Experience of the user using the interface.
CPU and GPU testing mainly relate to drivers and OS calls.