Well, I don't actually think most forums end up being active enough to warrant performance optimization being your first criterion for selection. I still recommend keeping usability and anti-spam features as your primary discriminants.
But if you do want to think about performance, as I hear it, any analysis should include three dimensions:
1. Read performance - driven by how many pageviews your forum is getting
2. Write performance - driven by how many new posts you are getting
3. Search performance - driven by how many searches you allow
You need to decide whether you need a dynamic forum or a static forum. In a dynamic forum, each view is generated by querying the database of posts (modulo caching). In a static forum, each post regenerates static pages representing threads.
Search is one of the most database intensive features of a forum. Most forums have at least some forum of search-limiting features to prevent the forum database from grinding to a halt. Google search does take the load off of the database but is limited to pages indexed by Google.
Which brings up another topic: making sure the search engine crawlers don't bring your forum to a grinding halt.
Anyway, I don't intend this reply to be comprehensive, but just wanted to start you thinking about the issues.
In practice, I would just go with one of the standard forums and tweak from there. Caching and database optimization goes a long way as evidenced by the this listing of large furums.
BTW, I'm currently enamored of SMF among the free forums. My first love was vBulletin, though.
If you want to continue this discussion, we should probably move it to a separate topic rather than digressing this one any further.
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