HHVM is used by Facebook to serve billions of web requests per day. To date, HHVM (and its predecessor HPHPc before it) has realized over a 9x increase in web request throughput and over a 5x reduction in memory consumption for Facebook compared with the Zend PHP 5.2 engine + APC.
It has improved at lot. It now has FastCGI, and Authoritative Cache. In Authoritative Cache, HHVM won't check cache is expired or not. Cache will be expired when you change your PHP file. On Production site, you won't update PHP files now and then but it makes HHVM faster.
On official website, we got Getting WordPress running with HHVM. You may find so many interesting posts there.
Here is another interesting comparison by Alessandro Tagliapietra on PHP, Nginx with OpCache, and HHVM. .
|Server - concurrency||Requests per second||Time per request (ms)||50% req served within (ms)||90% req served within (ms)||99% req served within (ms)||100% req served within (ms)|
|Nginx 5.5 (OPcache) - 10 user||7.39||1352.937||1277||1598||1755||1755|
|HHVM - 10 user||182.67||54.743||53||81||109||134|
We see that HHVM is many times faster even with PHP 5.5 and OPCache. PHP 5.5 is faster than PHP 5.3.