If your logons are very slow, it is most probably because your DNS is not correctly configured, make sure all of you clients use your internal DNS server, or if you cannot make sure other dns servers have forwarding set up to you main internal dns server (if you only have one server this should be your domain controller).
Then on tour DNS server you picked, set that up to forward all queries it cannot handle to an internet DNS server (like your ISP one, or council one or public ones like openDNS), or you could set it to forward to you router.