[ByteJockey]

> but I feel like it makes it very difficult to reason about performance

This seems like a form of premature optimization.

I mean, you're not wrong technically, but if you were exceptionally performance concerned, you probably wouldn't be using ruby.

If you're just regular performance concerned (i.e. you want to stop pathological behavior in the hot loop), you're really only concerned about 1-2% of your code. You wait until it's an issue, run a profiler over your code and optimize the couple of parts that are causing the issue.