Something isnt right, im getting 242 FPS on serverside running on a 1.6ghz passivly cooled tranquil pc intel nuc... You should be flying! Check all your FPS variables ( fps.limit )and make sure there is nothing limiting it ( Empty FPS ) etc.
are you manually setting up the server with steamcmd or are you using out of the box with LinuxGSM?
LGSM has CPU usage on the ./rustserver details
and for general debian you can run "top" or grab sysstat (apt-get install sysstat) then use mpstat or sar...
here have a link https://www.cyberciti.biz/tips/how-do-i-find-out-linux-cpu-utilization.html