Interesting indeed. It seems the system it's using, searches 'tags' or a tag, to return a list of posts by. For what's on the page, it's slow.
However, I digress. It's definately doable in vB with enough time, but a lot of information would be stripped out, since you're essentially removing/replacing forumhome and all tabular data, and instead filtering posts to be displayed in a list, and then returning all the categories, on every single page.
Doing it that way however, should load
much faster than a regular BB (the only reason the linked site is slowed down is due to poor markup).
I personally don't have time for something like this right now, however this should give someone an idea of how to go about it. Oh, and if you get someone who truely knows XHTML and its guidelines & considerations, the end product should load extremely fast in comparison to most BB's, and the one you linked