In every single one of these cases, it begs the question whether "JS" is the problem or "backwards compatibility". There are a huge number of inefficiencies that can be fixed with a full rewrite.

There was a recent conversation over ViteJS (a pure JS bundler) vs rust-based tooling, and when you dig into the numbers the real difference is SWC vs Babel. It raises the question whether a new transpiler written in JS can be competitive with Rust, but it's unclear if anyone tried.

Sucrase is written in JS and boasts the highest line throughput of any competing transpiler

https://github.com/alangpierce/sucrase

            Time            Speed

Sucrase 0.57 seconds 636975 lines per second

swc 1.19 seconds 304526 lines per second

esbuild 1.45 seconds 248692 lines per second

TypeScript 8.98 seconds 40240 lines per second

Babel 9.18 seconds 39366 lines per second