Apple TV / iMac Connection is SLOW

I've updated my iMac to El Capitan and updated my Apple TV. When I connect my TV to my iMac through sharing, it now plays a movie (on the iMac hard drive in iTunes) for a minute or so, then stops and the pinwheel just spins.

I've restarted both the iMac and Apple TV, didn't help.

Suggestions (besides waiting for an El Capitan bug fix) are greatly welcomed.