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.