He said the reason Japanese websites were so busy was the greater percentage of mobile-only customers, many of whom weren't using smartphones. You could go to a website on your cheap "feature phone" and still comfortably navigate a site that was mostly a sea of links, while the sort of site you see more commonly in the West requires navigating too many pages.
Smart phone penetration in China is still around 50%, so I wouldn't be surprised if the same logic applies.
EDIT: He also said in Japan programmers have about the same social status as the guy on the loading dock, so many of the corporate sites were programmed by fresh-out-of-college guys who would be passing that task off to someone else as soon as possible. Not sure if the same applies in China.