pretty much every script devteam/comunity says that the 'baby' is fast, light secure, etc.
concerning speed if you belive my word, punbb is BY FAR the fastest.
I sugest you test it, no need to make any special complicated tests. Just browse it, the difference is huge.
I gave a try to phorum ( "it's like fast and stuff" ) , it's a similar aproach to punbb, not bad at all, but punbb is beter. I run punbb and phorum in the same server, no ofense to phorum, but is way slower than punbb.
other advatage is the code: it's super easy to follow because it's simple. I get annoyed with all those fancy ways of code that nobody understands because they are suposed to be advanced and dificult. With punbb you easly follow the code execution and it's damn simple to code hacks for it.
also: it ouputs Valid strict XHTML and CSS, google gets all crazzy with it. Keep it valid and google will keep up snapshot of your site on a weekly base. Once again, make your tests, the differences are surprising.