The way Cloudflare works is it caches the content of your website at the data center to which a request was made.
This way, if your visitors are routed by the ISP to Japan, they will very likely be served a page from one of Cloudflare’s Japanese data centers. Once as second request is made to the same data center for the same file, and within its TTL, the file should already be available from cache.
You can test which data center your visits are routed to by visiting :
https://example.com/cdn-cgi/trace and substituting your domain for
example.com. You may want to try that with different ISPs, such as your landline, as well as your mobile internet service providers.
You can also try Argo to see if that benefits your domain somehow, though I’m not sure how effective it is in China, though perhaps @cscharff could shed some light on this. Argo is a paid service billed on usage.
You may also want to have a look at this article about the two possible options in China (Cloudflare vs partner) and a suggestion that you may use each for different domains: