Yeah, though this feels artificially slow to me. Even the earliest Teletype machines could manage 10+ cps, and by the mid-70's time frame (v6 was released in 1975) much faster devices were available (and of course video terminals were starting to arrive too).

I'm sure someone used a PDP-11 with a terminal this slow, but it's unlikely to have been the typical developer experience.

I used a PDP-11/70 with ASR-33 TTYs as late as 1978. They were still common because while slow and noisy they were much cheaper than the DECwriter, and could also read and punch paper tape. Since mass storage was very expensive (10MB for $20,000) and since floppies were not yet common, paper tape was the USB stick of the time.

Not because that's the system call to eat a carriage return?

I'm afraid not; there wasn't a system call to eat a carriage return until V7. Best I can offer for V6 is stty(2) with mode=|020.