Garrys Mod Server lags

From 20 users it begins to lag… On my Gmodserver can you Help me?

Dates of my Server:
RAM: 5 GB
Cores: 4 Cores ( 2.10 GhZ)

There is not much used of the resources…

My Server.cfg
logging on
hostname “[GER] Jedi vs Sith | Easy Rankup | Anführer gesucht!| SGC”
rcon_password “asdasd123123DE”
sv_password “”
sv_region 255
sv_lan 0
sv_logbans 1
sv_logecho 1
sv_logfile 1
sv_log_onefile 0
sv_noclipspeed 5
sv_noclipaccelerate 5
//Sandbox Settings
sbox_allownpcs 1
sbox_godmode 0
sbox_plpldamage 1
sbox_playergod 0
sbox_noclip 1
sbox_maxprops 750
sbox_maxragdolls 10
sbox_maxnpcs 10
sbox_maxballoons 10
sbox_maxeffects 50
sbox_maxdynamite 10
sbox_maxlamps 20
sbox_maxthrusters 30
sbox_maxwheels 20
sbox_maxhoverballs 20
sbox_maxvehicles 6
sbox_maxbuttons 20
sbox_maxsents 20
sbox_maxemitters 5
sbox_maxspawners 3
sbox_maxturrets 2
//Server Settings
sv_minrate 0
sv_maxrate 30000
sv_minupdaterate 66
sv_maxupdaterate 66
net_maxfilesize 30
decalfrequency 10
//Execute ban files
exec banned_ip.cfg
exec banned_user.cfg

You use 66 or 33 tickrate in you server?

I own a TTT server and 66 tickrate with 20 people is very difficult to put up with the machine you own