I have recently tried out to run our huge, about 10 years old application using HHVM. Interestingly enough, aside of few 5.6 features it didn't support yet, the process was surprisingly painless.<p>However, HHVM was generally between 2 and 5 times slower than vanilla PHP at rendering pages. In micro benchmarks it was way ahead, but running the whole application was a very different matter.<p>I decided to wait until the thing matures a bit more before trying again and then hopefully with more time to investigate the causes of this huge difference in performance.