Sites like webpagetest allow users to enter a website url and a test location, to run a speed test on the site from multiple locations using real browsers. Can anyone give me a basic idea of how sites like this work? You also have plugin's like Aptimize latency simulator or charles web debugging proxy app, that simulate the delay while accessing a site from different locations. I am assuming since these are plugin's these function in a different way. How do these plugin's work ?