Program design in the Unix environment (1984) [pdf]
harmful.cat-v.org
harmful.cat-v.org
This is one of many times when Rob Pike has complained about non-Research Unix being "impure" (and also VT100s suck and the Blit is the pinnacle of GUI design). He's right to a point, but an OS that enforced purity wouldn't have had nearly the success Unix has had. Most people don't want pure, they want useful, and purity can help usefulness or get in the way.
The speeds mentioned in that time and context are more likely for hard-wired terminals rather than for modems.