As a follow-up and some more technical background info, here my Mac specs:

iMac (Retina 5K, 27-inch, Late 2014)
3.5 GHz Intel Core i5
8 GB 1600 MHz DDR3

Display: AMD Radeon R9 M290X
Type: GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 2 GB
ROM Revision: 113-C408A0-730
VBIOS Version: 113-C408A0-017
EFI Driver Version: 01.00.730
Metal: Supported, feature set macOS GPUFamily1 v3

Display Type: Built-In Retina LCD
Resolution: 5120 x 2880 Retina
Framebuffer Depth: 30-Bit Color (ARGB2101010)
Main Display: Yes
Mirror: Off
Online: Yes
Automatically Adjust Brightness: No