How to turn off retina resolution in iMac 5k?
The retina display causes any somewhat graphic intensive apps to be laggy. It's a major design flaw. You can fix this by right clicking on any app and clicking "get info". Then click on "open in low resolution". 

You'll be surprised to see how fast apps run now. It will look not as clear, however. The retina sucks. My 2011 macbook ran everything better because it didn't have a retina display.