What in the world are you talking about?
This reads like someone who doesn't understand how to use a tool so they think it is broken. I've worked on Rails apps that outperformed Spring/Java apps routinely and node apps routinely at very large scale. You're very rarely bottlenecked on CPU in a modern CRUD app, much more often, it is the database.
It sounds naive to call the framework that runs github, that almost everyone in the world pushes their code to "veeeeery slow" and I don't know what you mean by "bad at io" in any context where that is a problem for Rails.
It's possible to make Rails relatively fast.
But Ruby is one of the slowest languages around. And Rails adds a lot of runtime overhead to that. It really is very slow out of the box.
And that matters for virtually no-one¹, because almost no-one is running their rails app at that scale.
That something would outperform Spring is really due to technical choices, the stack, suboptimal use or libraries. Because Java is undeniably multitudes faster than Rails; in everything.
¹ This used to be my strongly held opinion. But I'm shifting around since we're in a the middle of an unprecedented energy/climate crisis and a slow rails app is gobbling up electricity much more than a highly tuned one. Or the same service rewritten in Go or Rust or Java will.