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Indeed different geo-locations provide much of a difference with my website respose times.
You can use this free service to add page response times monitoring from around the world, over time.
I notice that for my AWS N. Virginia web servers the response times in East US is about 200ms (that's the best my website home page can response in), while in West US response times are about 350ms (increase of 150ms with every response time), 400ms from Europe, 500ms from the Middle East and about 800ms from South East Asia. That's a gold data, because you can assume that's (more or less) the latency all over the other websites. So if your servers are on East US and your friends are in europe, you can assume they gonna get a latency which is slower in about 250ms with every request.
You can try out this free service for yourself on: www.beatsoo.org
As already noted, there are other things that have more of an impact than physical distance from servers, but geographical distance does have some effect. Probably not much more than 10% of the end-user response time is dependant on distance. Most of the the time is used to download all the items in the page, i.e. Flash, .jpegs, scripts, stylesheets, etc. It would most likely mean quicker response times if you disperse this type if static content instead of altering your web application architecture. If you change your site to work in a distributed architecture, you might well have to replicate database transactions and sychronize the session state across server locations. Properly changing application architecture can be a significant challenge.