What Makes Admin Ajax or Domain Mapping Take 3 Seconds?

Hello All,

Recently during page load testing on sites across a WP 3.9.1 network of 50 plus sites, we discovered that every site suffers from an initial 3 second delay.

One consultant who reviewed the waterfall reports indicated (and the report identifies it) that admin-ajax.php or Domain mapping seems to be the culprit. Or is this indicating something else dashboard related?

I have been eliminating other plugins and testing various configurations to see if any of them had an impact on this 3 sec delay, but not seem to.

We are also running Multi Domains with about 10 domains active.

Is there a Domain Mapping or Multi Domains configuration option or setup setting that I may have incorrect that would cause this delay?

My questions is, how do I go about determining what is causing this 3 sec delay?

Any assistance would be appreciated.

Thanks in advance.