In reply to prayhumbly.

ecommerce should be cached until there is a sale. Even amazone is partially cached.
As for community sites, it depends on the exact nature of them but most of them obey the rule of 10% contributors and 90% lurkers. For the 10% participants it is more important to install object cache otherwise you will not notice any improvment from HHVM because most of the time will be used on the slowness of DB access

I have never heard of a wordpress site in which php was a bottle neck. In all real life cases it was always the memory and not the CPU power which the site ran out of. With memory usage of 100M per request to support 20 RPS for which you will need more then 2G free memory to handle so many requests at the same time, and 2G memory servers are not entry level servers and usually have more CPU power as well. But this is my experience and I would gladly learn about some case in which CPU did matter for wordpress site.