The average version of HHVM PHP virtual machine only has eight (8) weeks of support. This is not a very long time, so you’ll need to keep a close eye on your version as HHVM is not immune to security issues. Like any software.
Read more about HHVM release cycle and Long Term Support (LTS) releases.
WordPress is a stable technology which can not evolve very fast. Thanks to the evolving PHP and web platform we can apply improvements continuously. In 2015 the most talked methods to improve WordPress performance are HTTP/2 and the HHVM runtime.
Read more here: Improve WordPress performance using HTTP/2 and HHVM