Brimstone does try to use the minimal set of Unicode data needed for the language itself. But I imagine much of the difference with Boa is because of Boa's support for the ECMA-402 Internationalization API (https://tc39.es/ecma402/).
Author here - very cool to see this get posted! Thank you @ivankra for adding this to https://github.com/ivankra/javascript-zoo and running those benchmarks, I really appreciate it!
This started as a hobby project that I've ended up putting a lot of time into over the last three years chasing completeness and performance.
Brimstone does try to use the minimal set of Unicode data needed for the language itself. But I imagine much of the difference with Boa is because of Boa's support for the ECMA-402 Internationalization API (https://tc39.es/ecma402/).