UE4 performance on mac has improved significantly…but still trails quite a bit behind windows.
I have 27" 5k imac and it works acceptably… but it is orders of magnitude slower than my windows pc with gtx 980 and windows laptop with gtx 970M.
Windows laptops can run ue great, but you have to be very careful about the gpu in them.
GTX 960M and above.
Ideally, GTX 1070.