- Keep corpses OFF
(Ragdolls can cause lag in the long run.)

-Remove any Texture enhancements
(This is more for the larger ones, like Vfire and Stormfox)

-Remove mods you dont use or need here.
(Its easy to understand, the more mods, the more GPU)

-Get a better PC
(Gmod is a rough game for toasters, I suggest one of my laptops: Acer Nitro 5, a ez $600 gaming laptop)

-avoid NPC mods
(Weapons and NPC mods are consuming, refer to #3)

-Dont use physics, mainly bullet drops.
(It adds more props)

-Turn down Graphics
(The map is small, but for you toaster users, try that)

I cant really think of any important ones off my head, besides all that has been said.
Enjoy slaughtering combine.