Oh, teletypes were much faster than that. Here's a video of a restored 1940 teletype:
In manual mode, it prints out on the receiving machine as fast as you can type it on the sending machine. In automatic mode, using a punch tape, I clock it at about 6-7 characters per second.
It's worth keeping in mind that Gary Seven's "magic" typewriter in "Assignment: Earth" was actually a real piece of office equipment from the day, a forerunner of a modern printer or copier, which could automatically type out multiple copies of a pretyped text using a punch tape similar to what's in the video (rather than being voice-activated as in the episode). So the speed at which we saw it typing out Roberta's words probably reflects the speed at which such a device could actually print something out at the time.