HHVM has significantly changed the way PHP sites are optimized for performance. Right now it is probably the fastest stable execution engine for PHP scripts. However, with PHP 7 on the horizon things may change and HHVM may face a serious competitor as according to some benchmarking tests and those disclosed by Zend, PHP 7 is much faster than HHVM.
Important: Even though it's stable, PHP 7.0 is still in beta and it's not recommended that you use it on live websites. In addition, it does not yet have Memcache support which means that if you want to use the SuperCacher and PHP7, you need to disable the Memcache option from the plugin backend before switching to PHP7.