External Display Lag
I'm having this same problem on a 2015 MacBook Pro (13" / 2.9GHz Intel i5 / 16GB RAM). Oddly enough, I am not having this problem on a lower spec 2013 MacBook Pro (15" / 2.3GHz). I have more CPU available on the newer machine, but it's the only one with the problem.
My retina display is totally unaffected, and my computer runs as normal, but the external display (connected via HDMI) lags considerably. I've read that this is due to CPU, and that does not make sense in my case, because the older machine with lower specs and nearly no available space on the drive works just fine (no lag at all). Usually, I'd just use the old machine, but it barely works anymore, and isn't stable enough to do music production on.
MacBook Pro 13″, macOS 10.14