FPS problem on my Sandbox Server {Plz help}

I am currently managing a GM sandbox server & ever since I started it peoples fps will start to slowly drop when they join (This is for each individual client). To solve it this players leave & rejoin the server to make the fps go back to normal, then it will slowly go down and down. Whithin five minutes of playing each player can barly move at all. This is very very annoying and have not found any answers as to why this is happening. Can somebody please help me out.

Server IP: sb.hearthigen.com:27015

Remove the chat box.