I have been using LaTex for many years and I like it, but what's stopping other document editing software (like Word) to reach the same level of quality of results? LaTex has been around forever, but nobody else is close to that quality.
TeX underneath is really very speedy; on my several year old laptop it can compile several hundred pages in around half a second (and pdftex takes around twice as long).
I believe it's also possible to perform optimal line breaking algorithms incrementally, so you wouldn't have to reset the whole document as you add to the end of it (although you may as you add to the start of it, but that happens in Word anyway).