But like... something like a font renderer in Ruby? The thing that is incredibly cache sensitive and gets run millions of times per day on a single machine? The by far slowest step of rendering any non-monospaced UI?
The Earth is weeping my brother.
https://github.com/vidarh/skrift
(Note that this is a port of the C-based renderer libschrift; the Ruby version is smaller, but much less so than "usual" when converting C code - libscrift itself is very compact)