The session is generated when they access the site, not after the post.
There are several triggers (cache_builders/cache_trigger.php, libs/triggers.inc.php) that're checked and ran after a new post is made, but the issue wouldn't necessarily be the trigger, but the amount of data processed during a trigger, which with a large set of data and restricted resources could take a while; this is the case when more resources than needed comes in handy as it can handle a larger load with more resources.
