Changing the WordPress site language to any other than English slows down page generation times. A lot. This is caused by the slow and inefficient way of text domain loading. PO and MO files are designed to be used with PHP Gettext-extension. But since it’s an extension, it’s not installed by defalt on very hosting platform. To overcome this barrier, WordPress has re-implemented the whole MO file parsing in PHP, but WordPress’ implementation is quite slow and memory intense.
This plugin replaces the default MO-Reader with another implementation of MO parsing, which is faster than the default one. Only used text domains are loaded (since version 4.6 this is also WordPress’ default behaviour) and from these only used translations are loaded. It also makes use of hash tables included in mo files, which are ignored completely by WordPress’ default implementation. To boost the performance even more, the plugin also caches translations to the object cache, if one is installed. For optimal performance you need a fast object cache backend like Redis, Memcached or APC(u).
Fast Translate is a minimalistic fork of WP Performance Packs localization improvements. WP Performance Pack offers additional control and more optimizations for WordPress performance, e.g. use of PHPs native gettext extension, improved image handling and CDN support.