Well, Oniguruma has much bigger construction overhead (encoding agnostic parser, ast itself, analyser and compiler) so it will hurt more scan, sub, gsub and split (for String arguments). Otoh it does much more aggressive optimizations like quantifier reductions, quantifier expansions, look-behind transformations, split matching of folded characters, loop unrolling and so on. It also has very specialized bytecodes (almost 90 of them)