This has happened once before and they reversed it. But they said this last time too:
The discussions that have happened in various threads on Lemmy make it very clear that removing the communites before we announced our intent to remove them is not the level of transparency the community expects, and that as stewards of this community we need to be extremely transparent before we do this again in the future as well as make sure that we get feedback around what the planned changes are, because lemmy.world is yours as much as it is ours.
It usually is federated quickly within Lemmy itself. I can’t speak for kbin but in my experience on SJW, I typically get all the content from remote instances in real time.
I know there are some technical issues with the scaling of federation though, but hopefully that can be improved on.