As of yesterday we have released Mozmill 1.5.24. That release was a bit unexpected because we had already decided to not release another version of this branch. But we were able to include a bug fix, which let our tests run nearly twice that fast. And given that we cannot upgrade our Mozmill CI to version 2.0 yet, a quick 1.5 release provides us a huge performance win.
The problem in detail was in the local HTTP server we make use of, which is httpd.js. In July 2012 I recognized a drastic speed decrease, and figured out that this server takes about 6s to shutdown if the same locally served test page is used multiple times. Given that we still start and stop httpd.js in each of the tests, this sums up to a really annoying delay. As it has turned out it actually was a regression introduced by a change in Firefox and now after 15 months bug 777354 got finally fixed! So huge thanks goes to Patrick McManus here!
For more information about this Mozmill release please see bug 926909.