Basically the title. I want to drive around with traffic but with my computer. I can only really get 3 cars spawned in. The idea is that you can turn of certain lua features on cars to make it run better
Not really possible, the only option is to make traffic cars lower quality but that would take years, tons of work, and far too much storage space, and far too much work. This game is very cpu intensive. You should get another computer, probably with an amd chip because this game favors high core counts.
The biggest lag factor in vehicles is the physics. Not much you could do unless you were to use lower quality vehicles / physics structures, which as far as I know such a mod doesn't exist yet.
It's more realistic that you will upgrade your toaster before anything like that gets added officially. You shouldn't even have a CPU bottleneck with those specs and 3 cars, it's your garbo GPU failing to render that many cars.