El Capitan improves performance compared to Yosemite, just as Snow leopard improved on Leopard, but if you look into more than two generations of the OS the trend goes the other way.

I currently have two apple laptops, an old macbook that came with Tiger and use it to test beta versions of OSX. It's running El Capitan right now but is almost unusable.

My main computer is a rMBP that came with Lion. It worked better (faster, snappier) with Mountain Lion than it does with El Capitan.