[tardoe]

So I have seen this before - a lot of ISPs now days are using "optimiser" boxes that are designed to throttle Elephant Flows (https://en.wikipedia.org/wiki/Elephant_flow) to reduce overall consumption. Usually they add a little bit of buffering or the occasional TCP congestion notification to cause a client to back-off and (for example), reduce the streaming video bitrate. But I've also seen bad configuration that can cause this sort of issue - e.g. an mis-configuration that limits you to 2kbps vs 2mbps. The reason the Wireguard tunnel works fine is because it's UDP-based and you can't trigger the same congestion notification behaviour over UDP. These boxes are usually inline to your traffic and are often referred to as "middle-boxes" - more commonly they're used in mobile (4G/5G) RAN aggregation networks where bandwidth is more scarce but they're now being sold into fix-line network providers as a cost-cutting measure.