The purpose of flow layouts is to resolve exactly this problem at map movement time (e.g. user zoom or pan).

Google Maps already does some flowing, it's just not very good at it once you zoom all the way in.

If you have an important point of interest center-screen, then the lower priority labels flow away, or are priority pruned if overly numerous. This allows users to resolve invisible labels simply by zooming in.

I'm absolutely in agreement on the performance challenges, which are likely the real reason precomputed labels seem to be used on Google Maps.