Is that the reason the Microsoft Word team tells themselves as well?
We have multi-core, multi-gigahertz CPUs these days: there aren't cycles to spare to do this?
Zooming in a bit, Word also does not kern fonts as well as LaTeX, so it might be missing some logic there that would trickle down into more beautiful word spacing and text flow.
Yes, there can be edge cases where optimizing one section causes another section to be resized. My gut is that that is the exception, not the norm. More, for most of the resizes that will lead folks to do, it will largely result in a "card" being moved up/down in such a way that the contents of that card do not need to be re-optimized.
Yes, you could make this even harder by optimizing what the width of a section should be and flowing another section around it. But how many sites actually do something like that?
A lot of the time folks are sitting and thinking and things are idle. Perhaps Word could 'reflow' text in the background (at least the parts that are off screen)? Also, maybe the saved .docx could perhaps have hints so that on loading things don't have be recalculated?
To be clear, just because I would hazard that, does not mean I'm right. Would love to see some benchmarks.