The first, and easiest way is to go to Content > Site Speed > Overview. By default this will show you a chart of page load time over time.
First, to get enough data change the time scale to a full year. Underneath the date picker there is an icon with 16 dots in it in a 4x4 arrangement, with some dots filled in. click on that and move the slider all the way to the right. This will ensure higher precision and will capture some of the slower page loads.
At the bottom, in the 'Site Speed' section instead of 'Browser' select 'Country/Territory'. It will change the data from pages to countries. Now click on 'view full report' and you will get a world map with page load times.
It will look something like this:
The site I just did it on doesn't have enough data, but if you have a fairly popular site you should see a nice variation in page load times.
Google have a post about this on their Analytics blog with much better maps and more information:
http://analytics.blogspot.com.au/2012/04/global-site-speed-o...
Their maps look a lot better.
The other way of doing it is to create a custom report. I have one called 'Global Page Load'. Add 'Country/Territory' as a dimension, and 'City' as a sub-dimension. You may also drill in further by adding browser version, mobile, javascript, etc.
As metric groups, I have - in this order: DNS Lookup time, Avg Server Connection Time, Avg Server Response Time, Avg. Page Load Time.
This then gives you a pretty report where you can immediately see which visitors are getting slow responses, and you can further drill in to see what type of connections and which browsers or devices are slow. I was surprised that my light page, with compressed CSS and every static/cached was still taking ~20seconds to fully load from 30% of countries.
That is for pages that load, you need to add another tab to the report with error metrics to see who isn't getting through at all, and you would need to look at server logs to see who isn't even loading the Google Analytics javascript. All very handy, and eye opening in terms of the types of connections and the speed of connections that a lot of web users are on.
To many sites are guilty of having pages that are just far too heavy - like they only test from their 100Mbit city based connections. I am in Australia with a 1st world internet connection at 24Mbit and I avoid theverge.com desktop site because of the page load.
Edit: If anybody can work out a way to share custom reports in Google Analytics, let me know - I would be interested in sharing reports with others, for specific cases such as this.