Fastsync is NVidia's new "trick" for people who like to play on high FPS (no V-sync), but hate the tearing. It really does not help if your game plays below 60 FPS, which is what G-sync is used for.

Fastsync is software induced, versus G-sync which requires a special monitor with a chip built in.

CPU = i9 9900K, GPU = RTX 2080 Ti, Monitor = 32" 4K G-sync