On the one hand, yes, our computers ought to be able to handle text editing. On the other hand, as long as you meet the latency requirements one way or another it doesn't much matter whether you barely met them or utterly blew them out of the water by four orders of magnitude... instant is instant, to a human.
A browser is a tough place to build a text editor. You're running a huge, complicated, sophisticated text rendering and typesetting environment, which you're using only a vanishing fraction of but the browser doesn't know that and can't much optimize for it. (Some, sure, I'm sure it's got special routines for monospaced text, but it still can't know you won't just stick a picture in the middle of it a moment from now.) You're running on top of a fairly slow language, even after all the optimization work on it [1]. You're running in an environment that is deeply structured to be synchronous so if you accidentally write a for loop over anything that turns out to be larger than you expected, you've frozen your environment until you're done, to say nothing of accidentally handing the browser a big rendering job ("did you just open a directory with 20,000 files? here, let me render that for you..."). Any of these things individually might be overcome, but the combination is quite challenging. It's nifty that the browser lets you run "anywhere" but it is also in a lot of ways a crazy place to try to build a programmer-quality text editor.
[1]: I have to justify this every time I say it since somehow the idea that "Javascript is fast!" has sunk into the community, but it's not true. The easiest proof is asm.js... if Javascript was already "C-fast" or even close, it would not even exist. It exists precisely because Javascript is not a fast language. Javascript is much faster than it started out as, but it started out as an extraordinarily slow afterthought meant to run a statement or 4 in response to a mouse click. It has still stabilized on "much slower than C" and appears to have essentially plateaued. The result of speeding up something miserably slow by quite a bit can still result in something slow in the end.
