> (or whenever dial-up got "cheap")
Right, but that was not due to nntp, it was due to the bandwidth economics of the times. I ran a small site that only connected once a day when the phone call was cheaper. But if you have a permanent connection largely unconstrained on bandwidth, it'll be faster.
> This may, incidentally, help to explain why discourse on Usenet was generally considered to be superior to that, say, on Twitter.
But yes, that as well. When a response takes at least 2 days, there is an incentive to write well and thoroughly. The instant response chat-type forums of today encourage meaningless ping-pong responses.
> Also, Usenet was very much censored, in the sense that most sites would not even think about carrying most groups
This is a very fundamental difference between a distributed ecosystem like usenet and a centralized walled garden. A specific usenet site, as you say, might choose to not carry certain newsgroups. That is local control, not usenet censorship. Usenet as a whole still distributes it. If you want access you can just switch to a different usenet provider. You can also run your own provider! That's what makes it so wonderful. You are in control, not some single central site. There is no central site.