I have El Capitan and High Sierra installed on a different SSD. I try each of them alternately on the same hardware (i5 CPU, H170N-WiFi motherboard, 8GB RAM, Palit Nvidia GTX750 videocard). I can clearly tell High Sierra is more sluggish than El Capitan. For example, in High Sierra, I open Safari then I press Cmd+Q to quit it, and it takes approx. 1 second to quit Safari. On El Capitan, it quits immediately. When I watch Youtube videos on High Sierra, it may stutter (skip frames) occasionally, but on El Capitan, YouTube is perfect.
I will try the Nvidia-Update script. Hope it will resolve the issue.