PHP NG within 10-25% of HHVM performance
Zeev Suraski (CTO of Zend and therefore an extremely qualified authority on PHP NG) has written a fascinating article comparing the performance of PHPNG against the current HHVM versions. A must read.
Perhaps even more importantly, he has written about the current state and future of PHP NG.
For WordPress, HHVM 3.2 still holds an edge, however keep in mind that there are several ideas still on the table for PHP NG that might give it an advantage. But as it currently stands, PHP NG is still almost twice as fast as previous 5.x versions and far easier to install/configure than HHVM.