> I lived in China for 9 years and always found the internet, even for going just to Chinese sites, to be really slow.

Did you have your VPN / proxy on? That might be one of the reason as Chinese internet is only fast for traffic within its borders. Traffic that crosses borders are super slow in terms of throughput AND latency (if not blocked altogether). If you have your VPN / proxy on, your request basically crosses the borders twice before it reaches the destination web server.

Another reason I can think of is the mobile ISP incompatibility. For some ridiculous reasons, most "foreign" phones' (iPhones exempt) do not have full radio coverage when connected to CMCC.