I have a 5k iMac 27". i'm having trouble with 4k in 60fps, not very happy about it! Anyone else?? Even playing back 60fps 4k is a problem on my new shiny 2015 5K iMac 27". I was thinking it was my SD card that's too slow, but I'm using the Sandisk Extreme Plus 64gb which is rated 10, and...