But if you don’t already know a backend framework, Rails is probably not the best choice to learn at this point.
Learning a new framework has value in future projects. Definitely not for the product you plan to ship next.
I can't see any strong reasons performance or otherwise to abandon one modern framework (PHP, Node, Python, Ruby, Java, C#) for any other. Difference in cost is again minimal.
The best option is the one you know best already. Learning a new one always helps you grow.
Can't speak for OP, but it feels like every mature language/runtime out there has a framework or two that are popular.
Ruby has Rails: https://rubyonrails.org/
Python has Django: https://www.djangoproject.com/
Node has Express: https://expressjs.com/
PHP has Laravel: https://laravel.com/
Java has Spring (nowadays Spring Boot in particular): https://spring.io/
.NET has ASP.NET: https://dotnet.microsoft.com/en-us/learn/aspnet/what-is-aspn...
This can change with time, of course, but I'd say that it's hard to go with any of them, even Ruby on Rails is pretty good in most respects, as long as you don't try to fight what's idiomatic in it.
As for the performance, having a quick look here might give a very vague idea (though the code for those benchmarks is sometimes optimized a bit too much), though running your own is best: https://www.techempower.com/benchmarks/#section=data-r21&f=z... Also have a look at the filters for other stacks!
The JS ecosystem has Express, but that’s pretty lightweight. I think the sweet spot is probably leveraging SSR frameworks like Next and SvelteKit if you want something with “batteries included”