At one point HHVM was quite a bit faster then PHP until the release of PHP7. Now they are pretty comparable. In some cases you may see a performance boost and others not so much.

Here are some stats https://hhvm.com/blog/9293/lockdown-results-and-hhvm-performance

Some people use HHVM as their primary compiler and use PHP7 as a fallback.
https://www.digitalocean.com/community/questions/how-to-install-and-config-hhvm-with-php7-fpm-fallback-in-nginx

This is my current setup. It really comes down to what workload and task(s) you are trying to achieve.