DNS response time can be a huge issue. The type of software you use can also make a difference. I noticed a significant increase in the speed of lookups when I switched from using BIND9 to djbdns.
No, I didn't do any measurements other than subjective observations. This was on my LAN and using BIND9 I would see noticeable delays between the 'looking up' and 'connecting to' messages in firefox. After switching to djbdns you hardly have time to read the 'looking up' before it starts the connection.