Finally found some concrete proof that Optimus does reduce performance! I have been using quite a few gaming laptops over the years (MSI GE62-2QF, MSI GS73VR-6RF, and now ASUS ROG GM501) - the first two MSI laptops, I have occasionally tried to play games on external monitors and the performance is truly noticeably better than on laptop's internal display, but for quite some time I just thought it was majorly because the external display (usually a TV) employs some sort of built-in motion blur feature that just makes the gameplay intrinsically smoother, but apparently no, because when I'm using my current laptop (ASUS ROG GM501, which supports both GSYNC mode and Optimus, which in turn means I can choose to let the internal display be connected to NVIDIA GPU directly), I can clearly see a change in framerate between two modes, and that was only a laptop 1070!
Honestly, I just hope that NVIDIA put more effort into conserving battery life for dedicated graphic cards handling display, so in the near future we can just get rid of integrated graphics completely.