I run Rust on a mac and I am experiencing major lag with it. I play other steam games, such as Counter Strike Global Offensive, Team Fortress 2, etc. and I receive no lag, running at a good 60 frames per second. For some reason I get around 15-20 on Rust and it is very difficult to play. I had even tried setting my graphics to the lowest and I still experience major lag.
A) macs are incredibly underpowered and unoptimized for 3D games/applications. (play in windows)
and B) Half-Life 2 games are 8 years old
Rust is built on unity, and its much more multiplatform and server performance oriented.