Yes, but it may be worth effort. See for example this editor: http://evanw.github.io/sky/ and paste there even tens of thousands long text. It will work super smooth, like Sublime Text does. If only browsers provided low level APIs for text, font calculations and accessbility (what they already support internally), those kind of editors would make a lot of sense.

Sublime Text actually use the "same" * rendering engine (Skia) that is used by browsers (Chrome et.al) for the canvas. So there is no excuse for web based editors to be slow ;)

* https://news.ycombinator.com/item?id=11689481