performance tracking

Hi folks. I'm looking for some feedback and advise on a odd performance issue.

We have a Drupal 6 multisite that we've developed that currently houses 7 sites. The sites themselves are, as Drupal sites go, not particularly exciting or complex. It even has fewer modules than most sites we build. (All of the sites use the same modules and configuration.)

On our development server, performance is fine.

On the client's test server, performance is fine.

I'm looking for some tools to analyze page performance times. I'm betting some Drupal tools/modules already exist, but my searches are turning up empty. Anyway, I figured I'd post here before reinventing the wheel.