Even on modern hardware it takes quite a while to compile a latex document (seconds!). Word basically has to compile the entire document with every key you click in order to show you the WYSIWYG interface.
A lot of that, though, is actually loading LaTeX packages itself.
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).
Lyx is a WYSIWYG editor for Latex. It shows you quick typesetting in the WYSIWYG editor, but you can compile the underlying Latex document for beautiful typesetting.
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).