We use Amazon CloudFront as a CDN and I suspect that is causing the delays. We even see it cause delays in the US and Australia at times. A quick Google search shows plenty of other people experiencing similar problems. Apparently CloudFront routes requests from China all the way to the US, even to the east coast of the US in some cases. That said, our servers are in the US too.
Can you please run traceroutes to both dwz7u9t8u8usb.cloudfront.net and bitbucket.org then paste the output here? It will be interesting to see the route. Also, just to confirm please use your browser to double check assets loading from CloudFront are in fact causing the delays. You can do that with Chrome or Firefox's developer tools. I'm not sure if there's anything we can do about this other than switch to another CDN. We've considered that in the past, but it's always low priority since most people wouldn't even notice the difference. We do pay for premium support from Amazon, so at the very least we could open an issue with them with the traceroute you provide.
I also have this problem. The website is not slow although it is a bit slower than Github. The problem is the pushing and pulling.
For example the pulling, when I click "pull", it often shows processing for a long time (not yet show the enter password dialog at that moment). Then I abort, try again, and abort and try again. After retrying for three or four times, it finally asks me for the password to login, when I enter the password, the pulling/pushing is not very slow (although it's still a bit slower than Github).
It's resolved because the person who originally submitted this request never responded to answer our questions. We don't have computers in China to test with. I see you also opened issue #6178, so we'll follow up there.