There is a huge variety of available plugins for WordPress — 30,326 plugins as of today — and if you’ve tried more than a few, you’ll have noticed a marked difference in their quality as well (functionality, user interface, stability, etc.). If you know your way around PHP, you should take a moment to browse the source code of a plugin before installing it. You’ll notice quite a difference there as well. ;-) You can view WPSSO’s source code directly from WordPress.org’s SVN repository. If you do, please excuse the lack of comments — it’s on my To-Do list. ;-)
I’ve always kept an eye on performance, and used WordPress’s object and transient caches when possible, along with disk based caching when appropriate. NGFB and WPSSO are fast, but until recently, I’d never compared their performance to other plugins. As I prepare WPSSO v2.4.4 for release later this week, I took some time to double-check its performance and that of a few other plugins as well.