I think that would only work when the number of instances is small. Two solutions to this might be:
have instances act like relays where the home instance of a community notifies 10 instances and then each of those instances notifies 10 instances, etc.
batch updates on a timer such that once a minute all posts, comments, boosts, etc within a minutes are buffered and sent together.
I think that would only work when the number of instances is small. Two solutions to this might be: