Please see my other reply in this thread: "recaptcha.net" can also be used. Is that blocked in China too? I can't find a clear answer.

I pinged recapture.net and got a 50ms response time. Baidu would give a 20ms response time. That's on WiFi. That leads me to think the server responding to these pings is in certainly in mainland China, I think in Alibaba's IP range, but probably not a CDN. Interesting, thanks.