My forum is overloading on CPU usage?

[orange peel]

My website has been shut down twice for CPU overusage. This is confusing because I only have like 100 active members on my forum? Anyhow, Ive been told to minimize plugins to counteract this, which I did: I uninstalled a few. But now I have only the ones I want but its not enough since I only uninstalled a few minor ones. What else can I do?
I heard you can add plug ins that optimize performance like Cache plugins or whatever. If so, which ones are the best currently? Im v3.8.x

[Frosty]

You have 100 members active at a time or daily? If it's at a time, switch to a VPS server. If it's daily, then your Web Hoster is overselling, which means he's selling too much hosting packages and server can't handle so many Websites as it doesn't have enough resources.

[BirdOPrey5]

You have a bad web host if 100 users is over-taxing the cpu. Find a new host.