Yeah and the US is large. But New York city has a higher population density than Tokyo, yet only a fraction of the internet speeds. So while you can argue that there is a large area with no or low speed access, you can't excuse crap service in prime areas
It would be really interesting to see the distribution of ISP speeds by country. For example, the Akamai report shows that while South Korea's average speed is 29.0 Mbps, the majority (58%) of South Korean connections are slower than 25 Mbps. Thus, there are perhaps some very high speed connections that somewhat inflate the average. Absent some representation of the distribution, perhaps the median speed would be more representative.
It's also interesting to note that the study methodology is significant. For example, Akamai's methodology and results starkly differ from the FCC's findings, which estimates the average speed in the US at 31 Mbps in 2014 and quickly increasing.
The distribution of speeds is dependent not only on technical infrastructure but also on pricing. A very large percentage of Americans have very high speed internet access offered in their area, but many may choose lower speeds due to economic affordability or the realization that higher speeds are not needed due to personal usage patterns or the existence of speed bottlenecks aside from the ISP (e.g., internet servers, multimedia protocols, etc.).