To be clear, this is more zombies on the server, spread across multiple players, while improvements like disabling jiggle is client side.
If you as a client had 100 zombies around you, the overhead of animating, colliding and rendering them is still expensive and you would probably have a low FPS.
Must Fixes was started a few months ago, but that is just a guide. Multiple systems are still being worked on, polished and tested, which are not often in MFs. There is some overlap, like my vehicle damage task is marked MF, yet it is a system improvement task, not a bug. That one is basically done, but I'm not closing it yet as the values are still open to tweaking and I would like to work on vehicles smoking when highly damaged, if I spend the time. That could also be done during experimental, so not a high priority to me.