Cleaning Up The Code & Speeding Up The Site

We have a site that once we have higher numbers of users at the same moment recently is having some issues. The hosting company says we have a lot of high CPU usage on our very high-end VPS. They suggest we get work done on the site to hit the CPU less.

The site was built over a year ago and we have made several add-ons to the site since we rolled out the current version. What we are looking for is someone to come in and clean up the code for faster loading and hopefully lower CPU usage.

Must have references and examples of work.