I hope you're joking. You set it to full 2880 native resolution? Why would you think that would be a good idea to test lag? At that res you pretty much turned off retina display.
The computer is rendering the same number of pixels no matter how it is scaled. Raw pixel count is all that matters. Start adding monitors and *that* is where you see problems... although OWC hooked up three additional monitors for a total of four and said they were able to run 1080p videos flawlessly on each one... so I wouldn't even be worried there.
The only lag I've even heard talked about is scrolling resource-heavy websites quickly.... and even that is mostly mitigated with software updates and ML.