The tricky part is the user interface, text transformation (sed, query-and-replace, pipe through external programs...), text parsing (for syntax highlighting and code folding for instance) and invoking external applications, for instance to compile code or post-process your document. Web browsers are not particularly good at any of these things since they're not needed to purely "browse" the web.

If you want performance while handling big files you also need to optimize the way you stream data from the disk, something browsers are terrible at. Is there any web-technology-based editor that handles big files decently? Last time I've seen a benchmark it was pretty abysmal. Not that Emacs is great at it (it was pretty bad for a long time actually, especially pre-64bit computers) but it still was orders of magnitude faster than Atom for instance: https://github.com/jhallen/joes-sandbox/tree/master/editor-p...