I'd also like to state that FH works smoothly for me, and even did so on my first laptop I received when I was 12 that was low-quality and was only meant for light gaming. I have owned a couple PCs where it'll be a bit laggy, because as Dylan said, not every PC is the same, but I own a top-notch PC now, and even with FHHD installed on high settings and various other mods installed, it still runs like a dream.
It all depends on your machine, just like with every game.