Hello, my server running on 10-20 players on E3-1230 and 16gb ram and 50m bandwidth, But my server fps only 3-20 fps 

-What is the server FPS with no players?
-What is the server FPS with no plugins?
-When was the last map wipe?
-When was the last time you restarted the server?
-How long has this been a problem?

The cpu is not really suited for a big server i would have minimum of 4ghz i think but for 10-20 players it should work so its probably another issue with plugins/entity count/restarts

Sadly you are with SHIT host that offers SHIT (cpus) probably.

If you do not have FPS drops caused by map or plugins sadly you can't do anything. (please test this when you run vanilla server -> no oxide and no plugins and use a small 3500/4000 map) If you have good fps then a specific plugin or not wiped full with entities/just big size map can cause the issue.

If you still have in vanilla server low fps, idk change host...

You can't do anything about this because every host does the same trick. They buy DEDICATED Server with Intel Xeons most of the time because this CPU contains huge amount of cores (lets say 40 cores in this example) and they make profit later. 10 People buy a cheap Rust server because they split this dedicated server into multiple virtual servers or whatever they do. Then at the end they have a lot of customers per dedicated server but performance sucks. Rust Server Requires single thread CPUs but it's expensive to have a machine with I7 or I9 because as I know they do not have insane amount of cores to make 20-30 servers for 20-30 customers and only 2-3 people can use this machine as I know and prices goes expensive as hell... Correct me if I am wrong (personal opinion)