It is common to provide 'glue' records along with NS records, to spare DNS resolvers the bother of looking up the IP address of the DNS server itself.
However, it is not even desirable to provide glue in some situations (eg.
example.com NS ns.example.net ... it is not appropriate for a
.com nameserver to dish out information about
.net domains). In that case, there is a risk that DNS resolvers, lacking the glue information, will take longer to resolve the domain.
But how much longer? Is it worth defining my own
A record for
ns.example.com to point to the IP address of
ns.example.net (a so-called 'white label' or 'vanity' nameserver, such that I can provide glue? Or does it not matter much in practice?. Why not?