author of the library here. The native implementations have to follow the spec exactly, which means that they have to guard against specific edge cases. The reimplementations in fast.js do not follow the spec exactly, they optimise for 99.9% of use cases and totally ignore the 0.1%. This, combined with the fact that the JS is JITed straight into machine code anyway gives fast.js a significant advantage - it does less work and so is faster.