So I see two ways forward for the fediverse, either:
A. Mastodon has to be refactored / rewritten to use Ruby Fibers, the non-blocking concurrency feature for Ruby
OR
B. We have to switch to a different Fediverse software that is based on a more modern software development framework.
Personally I think B might be the best option, and my money is on GoToSocial!!
It's going to be a long time before GoToSocial is usable enough that it can see real-world load. But based on what I have heard about Mastodon outbox federation performance, I'm very confident that GoToSocial should smash Mastodon in terms of its ability to keep up with accounts that have thousands of followers from thousands of servers and post frequently.