How does WP perform with a large db?

Is there any realistic limit on (for example) the number of comments in the db before WP starts to suffer? A site I’m planning to switch to WP from an ezBoard system averages some 4000 comments a month, and a million hits every six months, so it’ll generate a hefty volume quite quickly. Does WP work ok at this kind of scale, or is it really only for one-to-many blog formats?

I’m opting for WP on this because BBpress is barely started and phpBB 3.0 seems as far away as ever. It would also help to integrate a lot of other content from the site beyond the ezboard.