The problem isn't runtime implementation details, but the static analysis and compilation steps which devs rely on constantly. The tooling has had major performance issues at times (though in my experience it has improved consistently) and the experience of writing TS could be dramatically improved by having a language like Rust or Go powering it at the core.

If you meant parallel processes within the tooling, I suspect web workers wouldn't be the solution (they're part of the Web API). Node does expose a worker thread API, which they might already use in the typescript repo (I'm not familiar with any of their code).