Re: Question about rendering time
Here are a couple of other reasons why SMF might be faster.
* SMF doesn’t use php-gettext for translation, it uses an array of text that can be replaced. This method approaches the speed of a native gettext implementation.
* bbPress has a lot of functions to register.
* SMF has (auto-detecting) built in support for memcached, eAccelerator, MMCache, AlternativePHP Cache and the Zend Optimiser
* Most (all?) of SMF is written inside PHP, i.e. the HTML is in strings that are echoed out of PHP, this is slightly quicker than dropping in and out of PHP to output to the browser.
* Many bbPress plugins don’t need to load on every page, they could start with some sort of conditional that requests the current bbPress location and then just send a “return” when they aren’t required.
That’s all I can think of after a quick look.