I know there is a huge thread on this but am trying to avoid reading through thousands of replies. My 2015 13'' rMBP running OS X 10.10.3 severely lags when I connect it to an scaled external display. Resetting the pram only made the issue worse (could just be me thinking it got worse, haha). I'm not sure what else I can do. The lagging is so bad that when I have it scaled to 4K native resolution typing in iMessage will be sentences behind input.