Namecheap is a fabulous hosting provider. Love 'em. That said, highly-redundant DDOS-resistant DNS infrastructure is not their focus--they offer it as a free service with hosting, but it's best to cough up a few extra bucks and move your DNS to something more robust with AnyCast (DNS Made Easy, Route53...) when you can if uptime's important to you.

Check this out for real-time query speed testing: http://cloudharmony.com/dnstest

(And yeah, looks like GoDaddy is a solid "Test failed" still...)

Funny note after running the test a few times--the worst performing provider is the one whose salespeople contact me the most...