Joomla! now runs a fair proportion of websites, and it's interface obviously appeals to a great many users (over 30 million downloads as of April 2012). For big busy sites, however, the performance isn't always as good as it could be. It's not bad, by any means, but can certainly be improved upon.
To clarify: by busy, I mean numerous visitors all hitting at more or less exactly the same time.
Aimed at developers and owners of large Joomla sites, the tweaks in this documentation will help you improve the performance of your site. However, it should be considered advice, and not a step-by-step instructional, if your site is that busy, the database tweaks in particular may actually hinder performance slightly.
If visitors are reporting long load times, especially during busy periods, then these tweaks may be of use to you.