Hope I can help clarify the problem. As a Chinese internet user, I see the problem to be both about congested international internet routes and the GFW (will explain later). Adding a monitoring station here would definitely help.
As you can see in Tencent’s servers there are two Hong Kong servers keep getting much higher scores - because they have much better international connectivity. In the Hosting/VPS world many service providers targeting Chinese market will use words like “CN2”, “Asia-optimized”, “Direct route to China” to advertise, because the general international connectivity to main ISPs in China mainland is so terrible. If the monitoring system doesn’t have an optimized network to access any Chinese nodes, the situation is not likely to improve.
The term “CN2” usually means AS4809, a famous choice for better international connectivity to China mainland, but of course they sell bandwidth at a much higher price. Currently if one want his servers to stay in the pool and, at the same time, actually usable in China mainland, he would have to spend much more money on this topic only to workaround the monitoring system. The said Tencent Hong Kong servers are real examples, which have CN2 access to China mainland so the connectivity is actually good, but Tencent would have to pay much more for this.
I once set up a server located in China with AS9929 access (somewhat similar to AS4809) and got a high score in the monitoring system immediately. Unfortunately my bandwidth is really limited so it got kicked out almost instantly after starting to serve the zone. Options like this exist but are really expensive, that’s probably why we are left with this unfortunate state.
For the GFW part, I believe the thread by tomli (Overcoming Great Obstacle of NTP in China) still makes sense. Although GFW is a highly unknown system and we often mix up ISP level QoS as part of it. It remains unknown what caused the ultimate UDP drops, yet.
I am using a simple workaround for myself currently - “ntp.felixc.at”, a geodns (gdnsd) that keeps running simple ntpdate validations against Tencent and Alibaba’s servers and return them for requests coming from China mainland, and CNAME to pool.ntp.org for the rest. My NS servers are carefully chosen to always have a not-so-bad internet connectivity to China mainland. And yet some of the NTP servers are still getting kicked repeatedly, and manual testing turns out to be complete UDP drops.
Attaching a mtr screenshot to one of Tencent’s server. The international packet loss is always so high and this situation is not likely to improve in a near future:
