I've now installed El Cap, and as an anecdote (given the age of my MBP mid-2010), I find 10.11 running easily and efficiently.
My aging MBP has only 8GB of RAM, but this is not posing any issues; so, although I don't know precisely, Activity Monitor verifies El Cap is not running with a lot of overhead. I generally keep 3 apps (Safari, Mail, iTunes) continually running, and AM shows just a bit over half of the RAM being used.
Additionally, my MBP's processor is a vintage, 2.66 GHz Dual Core i7.
So anyone with an aging desktop / notebook that's not that much older than mine, I think it fair to say the upgrade to El Capitan is worth considering (even though some of its features will only run on current models).
