Moby Dick, at its slender 752 pages, is 1.2MB of text. You can save the entire text to disk on every keystroke on just about any system today and keep up with typing just fine.
Assuming you are actually dealing with text in a text editor, you should be fine.
If you have 100MB+ files, chances are they aren't actually text.
Of course there are other ways to do this work, but a text editor that can efficiently open and edit very large files (e.g. Vim) is a great tool for the job. I will put up with otherwise excellent code-specific editors/IDEs that cannot work on such files but my default general-purpose text editor is always going to be something that can.
I appreciate that the design and implementation of text editors is basically an art form (one I've dabbled in myself) in which simplicity has aesthetic value, but efficiency and flexibility are very important for an editor that's going to be used for real work.
Text editors should also take huge files into account and provide sequential reading from the disk to memory. Even on Emacs, I couldn't work with the file. I ended up fixing it and importing it to Postgresql. And then I spent hours on indexing the necessary fields :).
Because if you're just viewing, the "array of characters" representation is going to beat just about any other hands down, especially if you just mmap() the whole thing.