You can find posts from people who were hosting their own single user instance that went down because of a popular post that got federated across multiple instances.

I either understand something wring about it or that still points out to Mastodon being slow and badly engineered.

Like, the updates are per server not subscriber right ? That's at worst few thousand requests, all of them can be essentially served from RAM. Even if you get 10k responses to your comment, 10k responses within say 5 minutes is still only like 30-40 req/sec, that shouldn't be much even for Ruby