Slow search for 'View unread posts'

Since phpbb.com uses the 3.0.6-rc candidates there is a new option 'View unread posts'. When I click on the new feature the results are shown only after 7-8 seconds.

Even when I mark all posts as read and do a new search it keeps loading for the same time. I think the query is taking to much time and should be reviewed. Otherwise it will slow down huge forums.