There are layout consistency issues across browser and OS. The place I’ve seen this is page breaks changing between computers (or even zoom levels). Line length isn’t controlled solely by the browser, IIRC both kerning (letter spacing) and word spacing is determined by the OS.
I implemented something like Docs myself, but with the goal of total consistency of line/page breaks. Knowing about these issues I used SVG. In SVG text you can override line length, but you still need to have a value in mind, a reference to be calculated independent of user environment details.
Regarding performance, one issue I can speak to is the cascading changes to line breaks. This can be pretty brutal when making a change to the beginning of a long document. To get acceptable performance I had to partially remove the application framework I had been using.
I liked my solution and it worked quite well in prototypes, but I don’t know how it would have faired in the wild.