Lag?

Why do Garry’s mod lag at my computer (talking about singleplayer)?..
I got some decent specs, so I don’t think that’s the problem…
My specs are: AMD Phenom II X4 965 3,62GHz, 8GB ram (DDR3-SDRAM) Win 7 and Powercolor Radeon HD 5750 1GB GDDR5…

any help?

Installed the latest graphics drivers?

Got em a while ago, still lags though…