AJAX slow? Check DNS Lookup time

Struggling with slow response times on AJAX updates, I stumbled on 1-sec+ DNS Lookup times for each operation.Why? Turned out this was a Firefox-only problem, with sparkling performance in Chrome. So why Firefox? Turns out that the default setting is for Firefox to try an IPv6 lookup first, and then drop back to IPv4. Turning that off (good note here) made the world of difference.