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.
My main computer is the original 2012 rMBP, and it's way faster on El Cap than the original OS it came with. Throughout the first couple versions, I was hankering for a new laptop due to the graphics chipset not keeping up with the HiDPI display. But somewhere along the way Apple really nailed the graphics performance to the point I never even think about it.
There was just one dud of an OS in there (Mavericks? I think) where the machine would just bog down over the week and I had to reboot it every 2 weeks just to regain performance. That was improved with the next release, and now on El Cap I only ever reboot for security updates (two months or so of uptime)
OS X does tend to get more RAM-hungry with each version though, so I recommend everyone to max out the RAM when they can.
I used an old 12" PowerBook G4 running Tiger for years past end of support. I just loved the size and the keyboard. It was also user-maintainable. Battery, memory, hard drive all easily accessed. I still have it and it still works. I tried running OpenBSD on it and while that worked, performance was a lot worse than Tiger.
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.