FFI presents an opaque, unoptimizable boundary of code. Having chatty code like this is going to cost a lot. To the point where this is even a factor in much faster languages with zero-cost-ish interop like C# - you still have to make a call, sometimes paying the cost of modifying state flags for VM (GC transition).

If Ruby YJIT is starting to become a measurable factor (after all, it was slower than other, purely interpreted, languages until recently), then the same rule as above will become more relevant.