How do I get sponge to consume less CPU?

My server is often shut down because of CPU overload, I have been ruled out as a sponge problem, when the player enter the server CPU will be 100%, and this will often last for a while.
The CPU has been 40~50% for only one player. Is there something I need to configure to mitigate this? And the number of creatures on my server may be a little more, how to reduce it?

Can you tell us what your server specs are?
If your using a hosting company, what company/plan?

A small server of about 20 players, renting a server.

Plugin list / timings please