Page load times on any WordPress site – leaving aside hosting issues – is simply and basically an issue of how much code is being actioned to display the page. For example, taking a WP site with no plugins installed the difference between running it with a simple theme – such as the WP default and a fancy theme with lots of bells and whistles can be really rather staggering.
Selecting a well written and lightweight theme is always a good start and selecting well-behaved plugins is essential. We come across – almost on a daily basis – plugins that, for example, load up huge volumes of code, scripts and CSS on every page load – regardless of whether their core code is being used on that page or not.
When it comes to a forum plugin then making performance comparisons between SP and bbPress is not really possible. bbPress is a fine plugin but it’s whole philosophy is about being lightweight in terms of features. It is severely limited by design modelled much more on the older ‘bulletin board’ simplistic approach.
SP is the complete opposite of that where we try and offer a feature rich experience and this too is by design. We have always tried to offer levels of features found on professional, stand-alone forums but integrated within the WordPress environment. If you want a web-site that includes a fully-featured forum then that is what we have tried to achieve with SP. if you want a website that IS a forum then I would suggest phpBB or vBulletin or SMF -all of which are excellent.
If you want a lightweight forum within your site that is basic then choose bbPress.
There are some performance notes you could take a look at here: http://codex.simple-press.com/codex/installation/installation-information/performance-notes/ but really at the end of the day performance is largely about planning, resources and compromises..
