They will not get slower speeds, just deprioritized access. They are not guaranteed fast speeds … a big difference.
This makes it seems like they are at least given a fair slice of the aggregate bandwidth, just a smaller fraction than a prioritized user would get. If it was truly deprioritized you might get 0 bandwidth if the channel was already full with prioritized traffic. That might actually be the case, but that's much worse than just lower speeds.
Yes, it's limited service, from a very limited network, and it's there to be where previously there was nothing.
That said, there's a company deploying fibre in my area as we speak, so as soon as they finish running the cable to my door I'm gone. But they were crucial to my being able to work in the last few years.