sucrase

I haven't looked too much into the TypeScript compiler implementation, but my theory is they've just had more bandwidth to do little optimizations or maybe got lucky on some design decisions rather than it being something fundamental. To be clear, this doesn't include any typechecking, so Babel and TypeScript are pretty much solving the same problem here. I've actually been hacking on a faster alternative to babel/tsc for simple use cases: https://github.com/alangpierce/sucrase . It has a built-in benchmark (`yarn run benchmark`) with at least one example of where typescript is about 3x faster. So you could look at that config if you want one example to compare the two.