I've had almost the opposite experience. VPN sort of worked, but I could not open a single HTTPS connection. The VPN problems I had I could trace to a bad WiFi connection (I had to lower my MTU and it worked fine).
Now, on previous trips I experienced what you mentioned. It seemed really like there was some machine learning going on, and after using a VPN for a while the connection would get bad. But I guess it might not be machine learning, there might just be a huge number of humans watching your traffic - which would explain why it is so inconsistent.
The thing that worked best for me is just using ssh -D (on most days). Our workplace uses ssh a lot for secure communication with outside china, so that couldn't possibly be blocked without hindering our work (and I believe 'they' have no interest in that). So whenever I had to access something for work that was sillily blocked (argh gmail), I just used the ssh connection that was open anyway.
Actually this is classic daily life of a chinese netizen: you are never quite sure what the cause of your network woes is (not without spending time digging into it). Is it due to ISP QoS, or is it reset by GFW, or is it just mere network failure?
And what most ppl do when facing this? They choose a local service instead of Twitter, Facebook, Youtube, Google. See, censorship is only a part (though a vital part) of the grand scheme.
Now, on previous trips I experienced what you mentioned. It seemed really like there was some machine learning going on, and after using a VPN for a while the connection would get bad. But I guess it might not be machine learning, there might just be a huge number of humans watching your traffic - which would explain why it is so inconsistent.
The thing that worked best for me is just using ssh -D (on most days). Our workplace uses ssh a lot for secure communication with outside china, so that couldn't possibly be blocked without hindering our work (and I believe 'they' have no interest in that). So whenever I had to access something for work that was sillily blocked (argh gmail), I just used the ssh connection that was open anyway.