Intel HD 530 slow rendering (best build i had)

[wildwillow]

Yes I was thinking the same thing, because it's a 5K screen and I don't know anyone who's using an aftermarket 5K (I'm on 4K) maybe that frame buffer and EDID is 5K specific... Wonder if it's worth trying to emulate a 21" imac instead of 27"....

Thats what you are doing using iMac14,1. That is a 21.5" iMac. Apple never released a 21.5" with Skylake architecture. The latest is Broadwell(Intel Iris Pro HD6200) that uses a iMac16,2(retina) system definition.