I haven't done any comparison lately, so thanks for your info.
I have also noticed that Finder has issues in 10.11 and of course it seems heavier in general. For a moment I thought that this may be an issue with the older hardware not being well supported
(like the bottlenecs in 3,1 GPU performance after mountain lion) but you wrote that you have noticed the same behavior with a new 5k iMac...
Anyway, imho, I think that the OS X
(macOS now) has a noticed detoriation in quality as of lately. The yearly cycle has a great influence in this issue as it stopped a lot of the OS refinement, at least as we knew it in the past (i.e. leopard/snow leopard, lion/mountain lion etc) now the most of the interest is about adding more support/features for iOS things and services, not to mention the lower quality of some of the current Applications
(and the decrease of functionality for some of them too) like, Airport utility (vs the old one), Disk utility, Mail, App store, iTunes etc.
MacOs is still very nice in general, but unfortunately it has taken the route of the Mac line, now it is like a poor relative of the wealthy iOS family... but we will see how things will evolve.
Cheers
!