The latest enlightenment on performance details and benchmark tests of the Apple Retina 5K iMac shows that this time apple is coming with something very powerful. If we compare it with the 2013 version of the iMac, which is equipped with GTX 780M graphics.
Tested three different configurations of the iMac computers, which include
4c nMP 3.7 D300s = ‘late 2013′ Mac Pro 3.7GHz Quad-Core Xeon; dual FirePro D300 GPUs
4c iMac 4.0 R295X = ‘late 2014′ iMac Retina 5K 4.0GHz Quad-Core i7; Radeon R9 M295X GPU
4c iMac 3.5 G780M = ‘late 2013′ iMac 3.5GHz Quad-Core i7; GeForce GTX 780M GPU