Ok, so that means that the problem is specific to file:// I guess.
The only thing we're doing different for file:// URLs is that by default the cache is disabled. Not having to build the cache can save a lot of time, especially if you're fetching from a repository that has a lot of other projects in it.
Can you set "use-cache = True" in the relevant entry in ~/.bazaar/subversion.conf and try again?
I can try mirrorring the twisted repository here locally, but that's going to take quite a bit of time and bandwidth.