Pushing a rich-root loom as a stacked branch is very, very slow

I'm pushing a loom made from a bzr-svn import to Launchpad. Here's what's happening:

jml@truth:~/src/twisted/launchpad$ bzr push
Using saved push location: lp:~jml/twisted/launchpad
Using default stacking branch /~twisted-dev/twisted/trunk at bzr+ssh://bazaar.launchpad.net/%7Ejml/twisted/
[/ ] bzr+ssh > 777KB 1KB/s | Finding revisions

Which is rather slow.

jml@truth:~/src/twisted/launchpad$ bzr info
Lightweight checkout (format: 1.6 or 1.6.1-rich-root or 1.9 or 1.9-rich-root or dirstate or dirstate-tags or pack-0.92 or rich-root or rich-root-pack)
Location:
  light checkout root: .
   checkout of branch: /home/jml/repos/twisted/launchpad
    shared repository: /home/jml/repos/twisted

Related branches:
  public branch: lp:~jml/twisted/launchpad
    push branch: lp:~jml/twisted/launchpad
  parent branch: /home/jml/repos/twisted/trunk

Format:
       control: Meta directory format 1
  working tree: Working tree format 4
        branch: Loom branch format 7
    repository: Packs 6 rich-root (uses btree indexes, requires bzr 1.9)

In the working tree:
      1632 unchanged
         0 modified
         0 added
         0 removed
         0 renamed
         0 unknown
         1 ignored
       191 versioned subdirectories

Branch history:
     15218 revisions
      2838 days old
   first revision: Mon 2001-07-09 03:33:37 +0000
  latest revision: Fri 2009-04-17 11:00:13 +1000

Repository:
     15234 revisions

$ bzr info sftp://bazaar.launchpad.net/~twisted-dev/twisted/trunk
Repository branch (format: unnamed)0KB 0KB/s |
Location: ] sftp > 0KB 0KB/s |
  shared repository: sftp://bazaar.launchpad.net/%7Etwisted-dev/twisted/trunk/
  repository branch: sftp://bazaar.launchpad.net/%7Etwisted-dev/twisted/trunk/

Related branches:
  parent branch: svn+ssh://svn.twistedmatrix.com/svn/Twisted/trunk

Format:
       control: Meta directory format 1
        branch: Branch format 6
    repository: Packs 6 rich-root (uses btree indexes, requires bzr 1.9)
Branch history:
     15213 revisions
      2838 days old
   first revision: Mon 2001-07-09 03:33:37 +0000
  latest revision: Wed 2009-04-15 23:23:02 +0000

Repository:
     15221 revisions

The .bzr.log is full of this:
330.600 hpss call w/body: 'Repository.get_parent_map', '~jml/twisted/launchpad/', 'include-missing:', 'svn-v3-trunk0:bbbe8e31-12d6-0310-92fd-ac37d47ddeeb:trunk:19179' ('<email address hidden>'...)
330.600 163 bytes
330.928 result: ('ok',)
330.928 14 body bytes read
330.928 hpss call w/body: 'Repository.get_parent_map', '~jml/twisted/launchpad/', 'include-missing:', 'svn-v3-trunk0:bbbe8e31-12d6-0310-92fd-ac37d47ddeeb:trunk:19163' ('<email address hidden>'...)
330.928 163 bytes
331.256 result: ('ok',)
331.257 14 body bytes read
331.257 hpss call w/body: 'Repository.get_parent_map', '~jml/twisted/launchpad/', 'include-missing:', 'svn-v3-trunk0:bbbe8e31-12d6-0310-92fd-ac37d47ddeeb:trunk:19162' ('<email address hidden>'...)
331.257 163 bytes
331.591 result: ('ok',)
331.591 14 body bytes read
331.592 hpss call w/body: 'Repository.get_parent_map', '~jml/twisted/launchpad/', 'include-missing:', 'svn-v3-trunk0:bbbe8e31-12d6-0310-92fd-ac37d47ddeeb:trunk:19159' ('<email address hidden>'...)
331.592 163 bytes
331.920 result: ('ok',)
331.921 14 body bytes read
331.921 hpss call w/body: 'Repository.get_parent_map', '~jml/twisted/launchpad/', 'include-missing:', 'svn-v3-trunk0:bbbe8e31-12d6-0310-92fd-ac37d47ddeeb:trunk:19155' ('<email address hidden>'...)
331.921 163 bytes
332.249 result: ('ok',)
332.249 14 body bytes read
332.250 hpss call w/body: 'Repository.get_parent_map', '~jml/twisted/launchpad/', 'include-missing:', 'svn-v3-trunk0:bbbe8e31-12d6-0310-92fd-ac37d47ddeeb:trunk:19153' ('<email address hidden>'...)
332.250 163 bytes
332.876 result: ('ok',)
332.876 14 body bytes read

So far, it has been running for 750s