The best way I have found to answer these types of questions is to check response time to yahoo.com from a different location.

An easy way to do this is the webperf.org monitoring page.

They also offer source for their web response time monitoring agent (written in Perl) , which would allow you to break down the response into dns lookup time, first byte received, and page completion from your site.

By coincidence, they happen to show historical response times for the yahoo home page on the webperf.org home page

Good luck..