A recent alarming network problem while trying to observe remotely at Green Bank brought back memories of the age of the 2400 baud modem, when you could often type faster than the system could display your characters. Fortunately I use the text editor vi, which was designed for these conditions. In fact, in this light some of the user interface decisions make a lot more sense.
The most prominent weird UI decision is that the editor has two "modes": insert mode, in which you can actually type text and it goes into your document, and "beep mode" in which the letters on the keyboard all tell the editor to do something (and it beeps if that something doesn't make sense). For example, to move around your document, the keys hjkl move you left down up and right, respectively. Bizarre as it may seem, these keys are on the home row, so if you're doing a great deal of moving around — particularly common when programming, i.e., debugging — this makes a certain sense. (Plus vi users get so hardwired that these become perfectly rational movement keys in video games.) The decision to have an editor-control mode also means that you don't need to press the "control", "alt", or "meta" keys very often, which makes sense since these often didn't work or were inconsistently placed on old UNIX machines. (I spent a while switching between terminals, some of which had "Caps Lock" and "Control" interchanged, and to this day I disable "Caps Lock" on every keyboard I use.)
The use of ordinary (case-sensitive) letter keys for editor commands also means that there's room for lots of editor commands, and they're very quick to use. So, for example, to search forward, you use "/"; "n" takes you to the next match and "N" to the previous. But there's also "f", which searches forward for a single character. While apparently bizarre, when combined with the ability of commands like "delete text" (d) to take an argument, this means you can do "delete up to the next single-quote" with "df'".
All this makes even more sense when it might be seconds from when you type a key to when you see its effect. So if, for example, you're typing along and you realize "hey wait, that 'a' was supposed to be an 'i'", instead of pressing the cursor keys many times and waiting to see where the cursor wound up, you can compose sequences like "Fari$", which will search back to the a, replace it with an i, and return you to the end of the current line. It's a bit like being the blind Zen archer.
All that said, I must make a peace offering to those in the emacs camp: being written not so much later, it offers similarly complex editing functionality, with similarly unusual (but different) UI decisions. I used emacs for years, switched to vi for some reason or other, and have stuck with it through inertia, mostly. (Even in the ubiquitous text boxes on the web I find myself hitting Escape followed by strings of gibberish. I'm not quite hardcore enough to go figure out how to make Firefox use vi as its editing component.)