I was at the BBC working on a large Perl codebase which was a fairly typical Catalyst/DBIx::Class stack. It had thousands of tests which thanks to the foolish way we loaded reference data and pre-populated memcache etc. had the test suite running for 3 hours. As you can imagine, it was a nightmare for TDD and indeed for large merges (by the time you've run tests trunk would have inevitably changed). We never fixed it thanks to a combination of lazyness and a case of "it's legacy code, won't be around much longer" syndrome.

http://qwerly.com runs a tight and fast Node.js stack at the front-end with a test suite that takes less than a second to run (a few hundred assertions). I'll dedicate much time to the framework around it's tests and make sure it's never slow.