Well-optimized JS isn't the only point of operation here. There's a LOT of exchange, parsing and processing that interacts with the File System and the JS engine itself. It isn't just a matter of loading a JS library and letting it do its thing. Every call that crosses the boundaries from JS runtime to the underlying host environment has a cost. This is multiplied across potentially many thousands of files.

Just going from ESLint to Biome is more than a 10x improvement... it's not just 1.5x because it's not just the runtime logic at play for build tools.