modems in the 90s would still negotiate 300 baud connections depending on how good the phone connection was, which could depend on the weather. I think there also needs to be a full duplex loop from the terminal emulator to the unix host and back for the character to show up on the screen. And consider that the both the PC that the emulator was running on was a 90s PC, and the web server I think was a shared host at the ISP. Some vi commands also would trigger a lot of screen update action. It was very easy to type far ahead of what was on the screen. If I lost track of what I had typed, I would have to stand up and walk away from the keyboard for a sec to let the screen catch up.
I started using modems in the late 80's. My first one was 1200 baud. Never once did a modem negotiate 300 baud on its own, even in the worst conditions. Were you connecting through a tin-can with string?
Terminal emulators are not CPU intensive applications. I ran one on an Apple II (8-bit, 1 mhz) and it could keep up with at least 2400 baud. If you were refreshing the screen with vi, I could see it being slow.
I was mostly inserting tags at the start and end of paragraphs of with `I` and `A` and other repetitive markup where I might have to `I` and the down arrow three times and then something else. I'd also do a lot of `:` ex based search and replace.
The way the university dial up pool worked, and when I worked for an ISP for 6 weeks in 96, there would be a room full of phone lines and modems where the dial in would happen. Sometime you would get one bad line, and sometimes you would get one bad modem, and probably sometimes you would get a bad line going to a bad modem. To keep users from paying local tolls, you would have to have several locations for the modem pools. In the case of the ISP, Bill Blue rented garages around the county and had T1s or something run out the the garages.
I didn't say it was common to connect at 300 baud, or that the vi story had anything to do with 300 baud. I know I did connect at 300 baud more than once in the early 90s, and I that is when I found out that I could read usenet news at 300 baud w/o using a pager.