The fact that the speed-critical parts of your problem could be reduced to a "few, small bits" indicates that we're talking about entirely different things.
I was in a problem domain where writing the time-critical pieces in a different language meant ensuring that huge amounts of working-set data were available to that optimized code. Efficient access to that data meant storage in native format, which meant that supporting two languages would require a nasty I/O layer for the interpreted language. Not worth it.
And that's just for the problems where it was possible to consider using Perl/Python at all. For some problems, the memory overhead of Perl/Python data structures ruled out use on all but the smallest problems. Again, not worth it.
It's nice that the speed-critical parts of your problem were reducible to a few lines of code, but that's not often the case. For most real problems, I/O is a huge concern, and can't be dismissed just by pre-processing the input.
It's certainly doable, and definitely the right way to go if you want number crunching in a high-level language, but it's not a free lunch. It's certainly not a quick one-time conversion and then you can forget about it.
There is SWIG support for Ruby, as well as the Ruby FFI implementation and RubyInline, offering various degrees of simplicity, but none of them are hard. The Ruby extension interface is more work, but even that is fairly simple to work with. I've taken C libraries and wrapped them in Ruby in minutes using SWIG for example.
Overall I often these days choose to write in Ruby first and replace even for applications that I intend to write the finished version of entirely in C. Prototyping in Ruby and rewriting in C once I have a working version is faster to me than iterating over a C version.