My wild guess would be thermal throttling. Basically it's drawing too much power and producing too much heat, thus it's being throttled to below 2060 performance.So about the 2060 vs 2070MQ, here's a pic to illustrate what I mean:
At ultra settings at 1080p (which is the resolution of the Y740 and most gaming 15" laptops - some are higher, but 2160p seems extreme, and even then it compares better), we have the 2060 getting nearly 50% better framerate (~50 frames more). On a 144hz gsync monitor, I know which card I'd rather be playing the game with, and it's not the more expensive one.
Which is really weird to me. Why does the cheaper card perform so much better? And while this is more extreme than normal, it's still fair to say on average you see ~15 fps better with the less expensive 2060.