Test & Improve web server performance

I’m trying to improve the performance of a web site. Beyond addressing a page’s file size and html coding issues,
– what can be done to test a server’s performance?
– what benchmarks can be applied?
– what tools are available to test the server?
– what companies/consultants specialize in this?

A “very slow” speed rating from alexa prompted a need for an investigation into performance issues. My IT guy points to the size of pages. But I suspect that’s one of many possible causes for this speed rating, and I want to make sure we do everything we can to improve performance.