Optimization

Hey everyone! I am not sure if this is the right place to post this, but i have issues with server it lags, even the cpu is at 30% is lags.
I have tried to solve this about 2 months.
So any one know tips like what to put in server.cfg or at line.

Thank you.

Server OS: Ubuntu 64-bit
Processor: Intel Core i7 4770K 3.5 GHz, 4 Cores. Hyperthreaded.
Ram: 8GB
Game(s): Garry's Mod, known as GMod.
Start Up Command: ./srcds_run -console -game garrysmod +map gm_bigcity +maxplayers 32 -debug -ip 0.0.0.0 -port 27015 +host_workshop_collection MYCOLLECTIONID -authkey MYAUTHKEY +rcon_password MYRCONPASS
Admin Mods: ULX.

MYCOLLECTIONID, MYAUTHKEY and MYRCONPASS are my collection id and etc.

Please ask if you need more information.