Looking for Site Audit Company

Does anyone have any recommendations for good website analysis/audit companies?
I have a client that needs to analyze their site in terms of speed, html, server load, peak time usage, etc. Not stats really, but the issue they are facing is that the site is loading really slowly for clients and after analyzing the site in-house, they cannot figure out what the issue is.

Thanks for any feedback.
ps - they would prefer to work with US-based companies that are not single-person operations, but rather have been in business for a while and are veterans in this kind of work.