Surprisingly quick. It took me longer to blur out my email subject lines than it did for YMail to mark 8000+ mail messages as read (in a folder of 11k+).
You could say “it should have just done it in the background” but this thing worked better than any of my corporate email clients have with that amount of mail (I’m glaring at you Entourage).
New YMail is looking pretty good, and the new Yahoo! homepage is incredible. Definitely check it out if you’re not on the new mail version yet.