-u performance

[elijah]

What are the performance issues with bogofilter -u?

Given a mail server where load is an issue, my plan is to not use -u when
filtering incoming mail but instead nightly scan each user's Trash and
spam folders for new mail to add to the databases. We have virtually no
load at night.

I have this pretty much working (with retraining mistakes), but my
question is this: should I bother? How much time does the -u option add
to filtering an incoming message?

I apologize in advance if this is not the correct forum for asking this
kind of question.

Thanks,
-elijah

(p.s. bogofilter is the freakin bees' knees!)