faelix - The problem with the CN zone can probably best be told by individuals with servers in that zone. It’s my guess that the one monitoring server can have issues and is getting improper time, either due to the “great firewall” or just the vast distance and numerous network hops. In which case having a monitor server in China (and a few more geographically distributed around the world) would be quite helpful to reduce incorrect scores and dropping servers.
Adding servers to the CN zone that aren’t physically located in CN to me isn’t a fix at all. Yes, if a zone is under-served then load-balancing should be subsidized by adjacent zones and continue outwards as necessary to meet demand. But for China I can only assuming the network issues that outside monitoring servers inside the firewall would cause just as much of an issue for CN clients to receive accurate time from outside the firewall.
Excluding monitoring from CN servers is a very dangerous proposition and just asking for trouble.
I can’t comment on the DNS cycling rate as I have no idea how fast it operates currently. However IMO, when you have a reduced number of clients in a zone they should not solely be carrying the burden of the load. If the DNS server did some query tracking and basic statistics then depending on the amount of “bandwidth” available from servers in a country vs DNS queries it could mix in servers from adjacent zones to meet demand.
The geographic proximity between server & client IMO is important. Too far of a distance and thanks to random network delays odds are the packets you are sending are all for not when your server is excluded from NTP’s selection algorithm on the client. If it’s just a SNTP client polling a single server it makes do with what it receives.