Thread:Manual talk:Extension registration/Caching only via APC?/reply

It all depends on how many extensions you plan on using. If you just have one or two, caching with memcached/redis will probably slow it down (I haven't actually confirmed this, I'll do that tonight). If you have PHP >= 5.5, you'll need to install the apcu extension, which is included in HHVM.

The code is definitely flexible enough that it should be trivial to implement support for memcached/redis if the performance is good enough.