I might have a solution for the lag problem.
So I've had this problem with my Macbook Pro (Retina, 13-inch, Early 2015) OS X El Capitan, the lag was just horrific, switching fullscreens was a nightmare and everything just seemed to lag lag and then lag some more, I've tried tons of solutions suggested by many forums, from reseting the PRAM and SMC to trying to change the display's resolution and even tried to erase and reinstall the OS but unfortunately none worked, I've literally spent weeks trying to figure out a solution, until I started noticing that the lag was the worst during daytime and that it was an intermittent lag, and one day I was sick of how much automatically adjusted brightness was annoying so I just turned it off and to my surprise the lag immediately stopped, it turns out that that feature was the culprit and the lag appeared only when the screen brightness was changing automatically and now my Macbook works like a charm, so here's what you can do:
Go to System preferences - Displays, and uncheck "Automatically adjusted brightness" in the "Display" tab.
I hope this works for you as well.