TF2 Maps lag in Gmod

As far as I know, I’ve had this bug ever since I got Gmod. I don’t know why, but TF2 maps lag like Hell if I load them in Gmod. What I can say for certain is that it has nothing to do with textures, as I have two Gmod maps that use TF2 textures, and they don’t lag at all.

If anyone knows if this can be fixed and, if so, how, please let me know.