Using WordPress translated to your own language results in a surprisingly large performance cost. In my tests it took twice (or four times!) as long for an unmodified WordPress 2.8.4 start page to load when using a localized WordPress.
Translating a lot of text in an user friendly fashion is hard to do efficiently.
But there are things we can do to make it a lot less painful. Most of the time is actually spent when WordPress reads the MO files containing all text to translate and structures it into the l10n object to prepare for the translations to come. On each page load.
The obvious solution is to cache the resulting object. So I did.