Queue accepts occasionally time out due to huge non-SQL time in createMissingBuilds

Accepting a source upload (using either DistroSeries:+queue or PackageUpload:EntryResource:acceptFromQueue) can time out with several non-SQL seconds near the determineArchitecturesToBuild call in SourcePackagePublishingHistory.createMissingBuilds. This has only been seen on the Ubuntu upload of libreoffice 1:3.6.2~rc2-0ubuntu1, from which we have three OOPSes before a retry finally succeeded:

OOPS-4b29d695266f2054311ae5f37329e638
OOPS-bc3d500f772ea9e03cb98de14e919500
OOPS-eddb1503e8421232ad7713cdada76c44

All show the same thing. A several second gap between

  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/queue.py", line 635, in acceptFromQueue
    self._acceptNonSyncFromQueue(logger, dry_run)
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/queue.py", line 619, in _acceptNonSyncFromQueue
    builds = pub_source.createMissingBuilds()
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/publishing.py", line 632, in createMissingBuilds
    architectures_available)
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/publishing.py", line 619, in _getAllowedArchitectures
    self.archive.enabled_restricted_families]
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/archive.py", line 2029, in _getEnabledRestrictedFamilies
    family for (family, archive_arch) in families

and

  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/queue.py", line 635, in acceptFromQueue
    self._acceptNonSyncFromQueue(logger, dry_run)
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/queue.py", line 619, in _acceptNonSyncFromQueue
    builds = pub_source.createMissingBuilds()
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/publishing.py", line 640, in createMissingBuilds
    arch, logger=logger)
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/publishing.py", line 653, in _createMissingBuildForArchitecture
    arch, self.archive)
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/sourcepackagerelease.py", line 434, in getBuildByArch
    distroarchseries.distroseries, archive)
  File "/srv/launchpad.net/production/launchpad-rev-16061/lib/lp/soyuz/model/sourcepackagerelease.py", line 416, in findBuildsByArchitecture
    results = list(query.config(distinct=True))

This happens on appservers, so there's no P-a-s involved. determineArchitecturesToBuild is pretty cheap without P-a-s, so I have to wonder if the the dpkg-architecture subprocess call is giving us trouble (perhaps letting a big xmlrpc-private request take a lock?).