Earlier quoted context omitted.
Yep, replaced string with Int array and now it is much faster. https://gist.github.com/rookie/3d9a848c6fefacc4254da921da79b... It now runs faster (when compiled, optimized) than the python code. xcrun -sdk macosx swiftc -O gen.swift
Ooh yeah, 0m0.363s for 1000 iterations on mine. So the slowness was in the string manipulation and character conversion operations.
I've learned from years of working on making code more performant that the biggest code smells to look at first are string operations.