said by matrix3D:Probably isn't overhead. The drop in speed is most likely a direct result of the vast difference in latency between a Japanese client going to a Japanese server vs. a Japanese client going to an American server. They'd probably get sub 10 ms ping times to the Japanese server but 100+ ms ping times to American servers.
Well not really that, but their internal networks within the country are fast but their peers to other countries are slow. It's kind of like running an office building with 100s of computers, your network might be a gigabit but your companies connection is only megabits.
In other words, those gigabit speeds are only within the Intranet of their country/that provider.