I intermittently perform 3rd party speed tests using http://tools.pingdom.com on my server. Recently, I have started to see long yellow bars for some of the static content on my sites, like images and CSS files (my understanding is yellow bars measure the amount of time from the request being made to the serving responding with "connect"). Checking my historical test results, I confirmed that there were never issues with to-connect times in the past, and that they only started after a few new websites were added to my server. Hence I'm thinking that the delay is being caused by a capacity limit of some kind set in my nginx.conf.
How should I move forward to further diagnose - and address - this issue?
OS is Centos, NGINX is built from source, 1.0.5.