Make your DNS response times faster across all nameservers.
One thing all sites have to wait for is the DNS resolution time.
If you have a site that connect and downloads the homepage in 30ms waiting for 80ms or more of DNS resolution is not the better thing.
One DO nameserver (NS1) is fast (~10ms) the others more slow (~80ms) NS2 and NS3.
There is a comparation of the fastest NS http://www.solvedns.com/dns-comparison/2014/03.