> still getting irritated by Firefox taking forever to render a 10M email source code in plain text when it could do something smart and render/view only the viewport that's showing.

How do you know what's supposed to be in the viewport, especially if the font isn't monospaced? You have to 'render' the entire thing at least once - at least to the point of measuring the dimensions of text (not a cheap thing) and figuring out where you have to force linebreaks. And whenever the user resizes their window horizontally or does various other things, you have to do it again.

Your typical browser is already pretty good at this - not to the point some dedicated text viewers/editors are, but pretty good.