The Unity Engine is not the greatest of engines for gaming, plus Rust itself is far from optimized. Do a little research on the forums here and you'll find people with beast machines, that suffer low framerates, and others who have mid range / low range machines who get decent framerates. The game's performance is all over the spectrum.
Funny that I play with 13 different people (not all at the sametime) and none of us have any issues running Rust; with the oldest computer in the group being a premade Best Buy computer that is just about 5 years old and the newest being a week old custom made BEAST. So that a pretty wide variety of computers, people, internet connection, etc and none of us have problems.
Yeah I know the game isn't fully optimized properly, but if you are running at 10 FPS it's more then likely something on your side and not the games side. Are you playing at max settings? Are you playing with grass on?