Caching slowness

I installed authcache as I need to provide caching for both anonymous and authenticated traffic. The core drupal caching module generates this page in about 0.8 seconds, so I was looking for something similar with authcache, but it actually takes 5.6 seconds (slower than with caching turned off) to generate the page.

Environment
PHP 5.2.5
Drupal 6.8
Engine APC

Authcache debug
Authcache.infopage_render: "5638.24 ms"
cache_render: "n/a (first request?)"
cache_uid: 0
cache_time: 1241532537
node_author: "Test"
(page_age): "1 seconds"
HTML/JavaScript time: 205 ms
Authcache.ajaxRequest (sent)q: "node/2274"
statistics: 1
Authcache.ajaxRequest (received)
HTML/JavaScript time: 762 ms

I notice in the debug info that the cache_render variable is empty, so I'm wondering is the caching actually working? I have looked through the modules code and see no reason for the response times. I have added a timer to a number of steps and can't see anything suspicious. Has anyone else had speed issues?