Well-optimized JavaScript can, if you jump through hoops like avoiding object creation and storing your data in `Uint8Array`s. But idiomatic, maintainable JS simply can't (except in microbenchmarks where allocations and memory layout aren't yet concerns).
In a game engine, you probably aren't recreating every game object from frame to frame. But in a compiler, you're creating new objects for every file you parse. That's a huge amount of work for the GC.
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.
Note that games are based on main loops + events, for which JITs are optimized, while compilers are typically single run-to-completion, for which JITs aren't.
So this might be a very different performance profile.
*edit* I had initially written "single-pass", but in the context of a compiler, that's ambiguous.
In other words you write asm.js, which is a textual form of WebAssembly that is also valid Javascript, and if your browser has an asm.js JIT compiler - which it doesn't because it was replaced by WebAssembly.
