This is sad but true. I have explained it on 9fans before, but I cannot find the messages. Fossil's block locking needs to be reworked some so that blocks waiting to be written (which are locked for that purpose) can still be locked and copied. I think a simple read-write lock might suffice, but I am not doing active fossil development anymore.
I should add that the length of wait is in fact proportional to the amount of data being flushed to venti, and that the bug was not recently introduced but has always been there. You saw a big delay because there was a lot to write to venti, not because fossil got worse. Russ
