I'm experimenting with El Capitan.
My platform is a late 2012 i7/2.6ghz Mini (quad CPU).
I'm running El Capitan from a firewire800 hard drive -- also experimenting with Yosemite (on another partition of the same drive).
Right now, El Capitan doesn't seem "as fast as" Yosemite yet, but I haven't tweaked it to the point that I have with Yosemite. But it seems to be "getting there".
I will make the disclaimer that I've made some "under the hood" changes with Yosemite that most folks would consider to be "unthinkable", but I've done them anyway and the Mini runs fine with them, much better than it did BEFORE I made the changes.
(Example: I've disabled VM disk swapping to reduce ins/outs to the drive)