Some gpus have functions like amd's chill that limit your fps intentionally, check if your nvidia driver has some jank like that, also check the affinity of your hl2.exe process, and make sure its using all the cores in your cpu, look if for some reason you've got vsync on or multicore rendering off, you should also try different directX values.

What are your temperatures like on your CPU/GPU? Use HWINFO64 to check. You might be thermal throttling. If its a prebuilt its unlikely to be overheating though

qs

https://steamcommunity.com/app/440/discussions/0/1698293255122561677/?ctp=3

hopefully this will work for you

qsd