How to turn off retina resolution in iMac 5k? I've started using many charts softwares recently. I open 5 charts at a time and my iMac 5k gets laggy whenever I do that. I checked with iStat Menus and it looks like my video card's ram is almost maxed out. Since my non-retina mac os computers work just fine with more than 5+ charts, I assume Retina resolution is taking too much memory.

How do I turn off retina mode and just use 2560x1440? 

I've tried pressing "option" key in Display preference, but it looks like it's still using retina mode.
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.