How slow can it be today? I did typeset one book using LaTeX and the tricky part was that it had to look like a series of books typeset with QuarkXPress, so it took me lots of time to modify/adapt LaTeX so that the output would match what the editor wanted. Later on I simply used QuarkXPress: not that I didn't like LaTeX but... I got a Mac with QuarkXPress.
But that was in the late nineties. Back when compiling, say, Emacs, took 45 minutes (which now takes 2 minutes or less depending on your system). Certainly LaTeX did benefit greatly too from advances in computing power during a quarter of a century? How can rendering a LaTeX document be still slow today?