The way you make a scripting language fast is by getting the hell out of it and into C or C++ as fast as possible, and PHP's library ecosystem embraces that harder than just about any other scripting language, is the reason (I think).

[EDIT] My point is mainly that Node's performance isn't really that impressive, in the field of "languages one might use to write server-side Web code". It beats absolute slugs like Python and Ruby handily, but past that... eh. And really, in its speed category you'd probably do just as well using either of those and paying a little more attention to things like network calls and database access patterns.