If I understand things correctly, this is venti catching up with the indexing that got suddenly interrupted when the machine was turned off. I can see this very easily by having a manual venti start and stop on another HDD, if I do stuff to that venti and shut it down immediately, I get the "indexing clumps" upon restat of that venti, whereas if I leave it for a few minutes before stopping, I get immediate venti startup.
I'm not sure if it is correct that venti blocks your system startup though. I thought it was meant to be able to serve immediately. But I propably don't understand too well. Robby