It is very hard to determine the response time of a website. If you measure it, the result will depend on the computer from which the measure is performed, on the page being accessed, on the hosting environment of the website and on a number of additional factors.
Why should you care? First of all, response time is important because web users are notoriously impatient. When a website is slow, they just go to the competition that is a click away. If you are paying ads and banners to bring traffic to your website, you are throwing money out the windows with your slow website. Secondly, since April 2010, the website response time is a factor in Google’s search result rankings.