When checking out a Bazaar branch, pip currently makes a full clone of the branch history. This is unnecessary and much slower than just fetching the latest revision:

For example, performance on my system for 'bzr co --lightweight lp:bzr':

0.60s user 0.11s system 5% cpu 12.234 total

Performance on my system for 'bzr branch lp:bzr':

65.41s user 1.48s system 39% cpu 2:47.91 total