I believe they used to benchmark via their own pool of test servers, but according to the sidebar they're now collecting "real user metrics" somehow:

> All results are based on RUM (Real User Metrics) data from users all over the world

> We gather and analyze more than 300million tests every day

> "RUM Uptime" shows the uptime of CDN providers as measured by real users. Due to bad Internet connections this number can contain false positives.

Agreed about Akamai, though — it appears to be clearly faster than those listed (they certainly had a HUGE head start). But also by far the most expensive. You get what you pay for in this industry!