A Microsoft file hosting and synchronization service.
I have the same issue. And, the response from Microsoft might as well have said, it's not their problem. My hunch is they intentionally slow download speeds, so copying and removing your content is substantially more inconvenient for the user, sustainably lowering their cancelation rate. I mean, it is the same reason you can't click 1 button to select everything and then select download, and you have to outsmart the system or individually select every item separately. While, these tactics are rather shady and distasteful, it is their platform so they can make it a slow as they want to. If I am wrong, I hope someone from mircosoft can offer a better explanation. My speed tests say i can download 100x - 200x faster than what onedrive is currently downloading at 1-2mbps.
The conspiracy theory doesn't make you look smart. It is complete and utter nonsense to think MS is throttling download speed to prevent people from cancelling their subscription.