At https://github.com/10gen/mongo/blob/53837263e1973f51b6960bafb618102d93eb1632/src/mongo/db/namespace_string.cpp#L412 we have a number of tests OR'd together for isImplicitlyReplicated(), but all of them begin with the predicate isConfigDB(). We can short circuit immediately if we begin with that test, since in the common case this will be false.