are there any obvious hogs at the top of Activity Monitor?
I just put 10.11.6 on my 2007 MacBook and at least in my case, AddressBookSync was a huge hog - I unchecked Contacts (and Calendar & Reminders) in the iCloud pref panel & things are much improved!
my admin user has iCloud completely off & its def fairly smooth (as an unsupported mac can be!)