Regenerate runs into a scalability wall with large node counts

Hello. I'm upgrading from a 6.x-2.x-dev release, from approximately the September 11th CVS commits. After moving to 2.0-alpha1, I'm trying to regenerate activity message - specifically "Record an activity message when: node insert types Mytype". The batch API fires up, gets the bar to 50%, and just stays there. It never finishes - just keeps going in a loop. I'm not sure if it ever successfully regenerates anything but it doesn't look like it does. This is with Pressflow 6.16.96, and I snapped down to a dev environment with all of the performance stuff disabled (i.e. no varnish, no memcache, etc.) Tested with both Firefox and Chrome.

I haven't dug into the code yet to determine what's going on but I thought I'd start with this post in case someone had an idea. Thanks in advance.