As a Visible customer, I can say the speeds are downgraded. It used to be that if you made it down to 5Mbps, you'd have to call them and your speed will go back up to ~25Mbps. The last few calls have resulted in no speed increases. Time of day doesn't matter so I don't believe their congestion story. They do allow for tethering one device. It's only $40/month and works out well if you just need a slow connection at some location.

That's true, MVNO and prepaid are often deprioritized from the start, while postpaid gets deprioritized only after consuming say 22 GB/month.

And the other trade-off is the "real customers" get to roam on other telco's towers (in some scenarios, depending on partnership agreements), while MVNO and/or prepaid often don't.