How do I make my website on Google App Engine accessible to visitors in China?
Assuming Google has, and routes to, Datacenters in Asia, the latency should be reasonable.
The reverse proxy to avoid the firewall should be in a country that does not censor and is as near as possible to the target area.
In those conditions, google would choose a datacenter near your reverse proxy, and the latency is rtt(google<->proxy)+rtt(user<->proxy)
But you really have to try this out.
China blocks appspot -- How do I get around this?
Assuming the censorship was not an issue, how bad are the latency issues?