Rails uses Ruby in a way that is defeats virtually 
    all best practices to produce a JIT-friendly code.

I've seen this mentioned a lot, and certainly the history of compiled Ruby + Rails benchmarks indicates this is true.

However, I've never quite understood -- why exactly is this the case?

Is it just the sheer size of Rails? Or is it (ab)using Ruby in weird ways?

edit:

This is the more or less definitive answer, I guess, though it's a bit over my head!

https://k0kubun.medium.com/ruby-3-jit-can-make-rails-faster-...