In wheezy (and all other current linux distros) there is a well-hidden bug that might freeze LVM if deleting a snapshot fails. Doesn't sound too bad one would think, but actually, creating and deleting snapshots is what buildds do all the time, and so they run into that issue quite often. It's worse on some architectures than on others, and so some packages are currently not built as fast as we're used to.

Details about the bug are in #659762. Anyone fixing that would (not only) do our buildd network a big help (and you don't need to know anything about autobuilders for fixing that).