On every pageload a localized WordPress site spends a huge amount of time setting up the translation (l10n) object. Startup for a localized site can be almost four times slower than an untranslated one.

This plugin stores the translation object between pageloads so it only has to be created once, resulting in significantly better performance (performance graph included with screenshot).

Plugin supports caching multiple languages (if you use a multi-language plugin).

Available storage: Plain file, APC (Alternative PHP Cache), or SHMOP (Shared Memory)