If you have a Mac and DNS resolution seems really, really slow — like, five seconds or more per lookup — check to see if you have an IPv6 address. You probably do.
Then check to see if you have an IPv6 DNS server listed. I bet you don’t, or it’s wrong.
Solution: add an IPv6 DNS server to your list.
Google DNS IPv6: 2001:4860:4860::8888 and 8844
OpenDNS IPv6: 2620:0:ccc::2 and ccd::2