Actually, Emacs is perfect over a 300 baud line. I have used that. The Emacs redisplay algorithm is provable optimal about sending the minimal number of characters over the line, as long as the termcap entry is correct. It was fun to watch redisplay in action over a 300 baud line, as you could see it draw the text, doing weird stuff in order to reuse the text already on the screen.
This came at a price of CPU though, Emacs required a full MIPS in order to work well. And a whopping megabyte of memory as well.