hhvm

HHVM can really speed up your PHP-based web site. Most reports are somewhere in the range of 2–4x faster. Unfortunately, HHVM isn’t very stable and will suddenly die, just of the blue, from time to another. Fortunately, if you’re running Nginx it’s really easy to set up PHP-FPM as a fallback.

Since version 3.9, WordPress have been 100% compatible with HHVM and I have begun replacing PHP with it on a few of my servers to experiment.