How to speed up the DNS?

[gliu]

I have juist set up a DNS(bind 8.23) server for one company.I fould it slow to solve the names that are not in the db of DNS but can quickly response to the correct ones.

eg
 ping a.b.c.d (a.b.c.d is no entry in db of dns)  very slowly cost about 1 min
 ping ok.co.jp (ok.co.jp can be solved by the dns server) very quickly

I think the root.cache file may cause the delay.Because the dns server is still used in the internal and can not access outside.Every query tries every server in the file.

My thought may be wrong.Can anyone give me some advice and teach me how to avoid it?

Thanks in advance.