Ruby is very slow so you gotta squeeze everything you can everywhere. Even a seemingly simple method will have to be scrutinized so that overall performance isn't impacted. It's death by a thousand cuts.
There's churn that comes with that. Ruby will have code that is ever changing to gain 5%, 10% performance every now and then. You gotta put that on balance: in a language like Go this method would've been ugly from the start but no one would've needed to touch it in 100 years.
See the commit that made it complex: https://github.com/ruby/ruby/commit/9b49ba5a68486e42afd83db4...
It claims 20-50% speedups in some cases.
There's churn that comes with that. Ruby will have code that is ever changing to gain 5%, 10% performance every now and then. You gotta put that on balance: in a language like Go this method would've been ugly from the start but no one would've needed to touch it in 100 years.