One of the problems is Japanese providers deliberately bottleneck the upload speed to 70-100KB/s. This is to slow down P2P soft like Bittorrent that chews up so much of their capacity. So it may be that the uploading request rate for the next packet of info is being stalled. I have 50Mb/s now, and was the 19th person in Tokyo to get ADSL (Mukojima was the 1st ADSL location in Japan at 2Mb/s) but I'm STILL bottlenecked after all these years.
Additionally, you'll find that XXMb/s download speed is only true if you have a multiple number of PC's all downloading simultaneously. So the bandwidth is "in parallel" not "in series". You can get round this by emulating multiple IP's on your PC if your using P2P software like Bittorrent.
Finally, XP has some reghacks needed to maximise bandwidth properly...