There was a time when hack was new and it was so much faster than php. PHP really got their act together (nothing like a little competetive pressure) and made the 7 series that ran at almost the same speed as hack. People are inertial and won't change unless they have too or something is significantly better.
I fear you are conflating two things: Hack is the language (type system, syntax) while HHVM is the runtime (which can run Hack code, and could PHP until very recently). Your answer on the speed pertains to HHVM.
