Strange as it is--or at least strange to me--my early-2008 iMac with 2GB of RAM is running better and faster now with El Capitan than it was with Mavericks--go figure. No clean install--just update from Mavericks--smooth as silk.
I don't know how exactly this works from the technical point of view. But once you make a TM backup with a newer operating system, the older system version refuses to read it as incompatible. So when I was installing Cappy beta, I switched Time Machine off BEFORE install (so that it had no chance whatsoever of making a Cappy backup).