My iMac, bought back in August 2007, is noticing the effect of newer OS X versions. The previous release, Yosemite, was clearly slower until a few incremental versions later fixed the memory issues so smaller systems with only 4 Gb memory could work comfortably. El Capitan, which is the internals tune-up for Yosemite, is definitely slower on older pre-2012 systems that cannot use Metal, the sub-system intended to offer a speedier computer.
There are some things that don't work correctly with this version of OS X, but I do like the disappearing menu bar at the top of the screen and the split screen option to set up and view two apps or files. Once loaded, the apps do run at the speed I have seen before, but the spinning beach ball does occur a bit more often.Maybe its time to buy a new system...
