In a Nutshell

The PHP version of FastTemplate is implemented as a native PHP class and uses regular expressions for parsing.  This means two things: the templating code must be interpreted for each request, and the template must be re-parsed for each request (or sometimes even more often).  By comparison, YATS is written in C as a PHP module, so it is always available.  Further, it parses the template only once into a set of stored data structures and uses these to quickly substitute new variables on subsequent calls to retrieve the template contents. The file is not re-parsed unless it changes or a new server process begins, no matter how many requests occur. Finally, YATS adds some additional features such as multiple nested sections within a template, and implicit looping within sections.

YATS >= 0.93 supports the GNU gettext library/tools and utilizes special tags to mark certain content for inclusion in the translation catalog. This allows creation of a single set of HTML templates that can be used to display your site in multiple languages.

Various iterations of this code have been/are running at Epinions.com and are sufficiently fast for the high traffic volume this site encounters, with several YATS templates used for each user http request. No specific speed claims are made. Your mileage may vary.