An interesting analysis by Justin Frankel, the lead developer of Reaper:
http://1014.org/?article=697
“TL;DR: Retina iMac (4k/5k) owners can greatly improve the graphics performance of many applications (including REAPER) by setting the color profile (in System Preferences, Displays, Color tab) to “Generic RGB” or “Adobe RGB.” (and restarting REAPER and/or other applications being tested)”