you don't have a good host server or what? i play on your server and i have allmost full fps / no lagg; some players have a bad internet connection and they are moving in "teleporting mode" ))); and yes this depends on a good configuration off plugins; about eating cpu/net packets/net bandwidth space - if you have a low bandwidth this may cause lag to players/ 15 players - 15 slots - more bandwidth and fast data packet sender to client side; some net packets drop it and that information never come to player = this make the screen freeze for a ms/short time; these errors can affect cpu and make it highter; are few zombie servers full of plugins installed and well customized - they don't have any problems with lag or fps;
when you write in console "developer 1" you will see you have an repeating error from a weapon, this flood the backgroung of the client console and eat some of his fps!