Mark wroteMostly it will be slower because it is written with PHP classes which are parsed slower than straight PHP spagetti code that most other PHP boards are written in. It's a trade-off. The classes allow Vanilla to be way more extensible, but slow it down at the same time.
But the problem arises that a lot of these queries are built on-the-fly with different options - something that you really need to be able to do. So I've left it like this until I can think of something better.