I’ve noticed that when I play Garry’s Mod, the game runs slowly, and jerkily (or rather, at a low FPS)… which I’d assume was because of my computer, and so on. But, for some reason, when using the camera it runs as smooth as any other Source game! Which brings me to the question - why is the HUD slowing me down? Why is the camera speeding me up? Why does this happen regardless of any replacement HUDs I use?
You could see if its the culprit by going through the console(~) commands and finding the command to hide the HUD.
cl_hud 0 or something like that.
Clean your gmod.