I have a 2011 27" iMac w/ Nvidia GTX 780M and a 3rd gen Apple TV. I cloned my Yosemite partition just before instilling El Capitan, so I have essentially two identical installations, just one running a newer OS. It seems like the upgrade to 10.11 is causing some major lag with Airplay Mirroring, down to about 1 frame per second. On Yosemite, I get something like 15 frames per second — definitely not fast, but watchable. Anybody else seeing anything similar? I've already uninstalled any 3rd party extensions that would interfere with AirPlay.