As a webpage becomes more feature-rich it often uses more DNS lookups which makes it render much slower.
This process can slow down your webpage considerably and one of the decisions you will need to make about your pagespeed is how many DNS lookups are too much.
A lot of sites with excessive domain calls are running multiple tracking applications that call back to the tracking site to send information.
If at all possible the number of separate tracking and analytic connections should be minimized, and if any of these are not really needed or provide information identical to other tracking that is being called, they should be removed.
Stuart MorrisExcessive Domain Calls12.02.2013