strange caching issues with pressflow 6.26

[jb044]

Guess this is more of a support request then an issue, however it is specific to pressflow, with vanilla drupal we don't have this issue.

For one of our sites we have this setup:

varnish
apache + mod_fastcgi + php-fpm
memcache
php-apc
pressflow 6.26

After a cache clear all form the interface things get really funky. Basicly a page request takes about 1 minute! After a while things normalize and the frontend starts to perform as it should again. There are some steps to increase this (load the frontpage, restart memcached, force a theme registry cleanup by visiting /admin/themes). We don't have this problem with normal drupal 6.26.

I'm having problems debugging this. I installed devel module and configured it to use xhprof on my dev server (same setup) but it shows nothing out of the ordinary, that is wall time is at most a couple of seconds not one minute or more. Please assist, as said the problem is specific to pressflow but to be fair also for this specific sites theme.