Stupidly low fps on a high-end machine

I have two 980’s and a i7 4970k, and on a prop & npc littered gm:construct it does not go above 40 fps.
The gpu usage is trash, it uses like 1-10% of both of them.
Is there a way to fix this or do I have to upgrade to pentium for the source engine to give me high fps?

You’re using a 10+ year old engine. It’s not optimized to use newer hardware to the fullest extent. Also, multi-core rendering is disabled in GMod.

The better pc you have the worse it runs lol.
10/10

No, it just depends how fast a single core of your CPU is.