After the update to Developer Beta 8 I have experienced shutter & lag (video, not audio) when watching Netflix (in Safari) or a movie in MP4/MOV format (1080p) with QT Player. Previously with Developer Beta 7 everything has worked just fine. I'm using a 27" 5k iMac.
El Capitan Dev Beta 8 & Slow Video Performance (MOV/MP4, Netflix)
You may want to file a bug report. Videos and NetFlix are fine on my 27" non-Retina iMac.