I'd say that our JS game engine codebase is generally idiomatic, maintainable JS. We don't really do anything too esoteric to get maximum performance - modern JS engines are phenomenal at optimizing idiomatic code. The best JS performance advice is to basically treat it like a statically typed language (no dynamically-shaped objects etc) - and TS takes care of that for you. I suppose a compiler is a very different use case and may do things like lean on the GC more, but modern JS GCs are also amazing.

Basically I'd be interested to know what the bottlenecks in tsc are, whether there's much low-hanging fruit, and if not why not.