I didn't dig into it too deeply at the time, but I think part of it was that you don't need to open and write to a socket, so that's avoiding some system calls (socket(), bind(), sendto(), close()). IIRC we had nscd set up so clients directly read from shared memory that nscd kept updated, rather than getting requests over a socket.

There's also probably some savings around not having to convert between the structures used by gethostbyname and DNS questions&answers.