FIX* I had the exact same problem. Lag on Youtube, desktop videos and Adobe Premiere pro playback. The answer is to reset the PRAM. Press and holding the Command, Option, P, and R keys right as you turn the computer on for 20-30 seconds. You'll hear the startup sound twice over this time. After the second startup chime let go. It will chime for a third time and startup. Carry on as normal. At least, this worked for my Macbook Pro 2015.