Does anyone have any idea how much performance is being left on the table by writing these compile-to-js compilers in JS? Could we get another 10x improvement by writing the transpilers in something like Go/Rust/C++? Frontend compile performance is beginning to get painful with large codebases...

For small code bases yes, or for simple transpilers. But for large codebases (like say, Google Gmail/Docs/etc) a global optimizing compile tends to be dominated by the O(f(x)) of the optimization passes, which tend to run in a fixed point loop.

If you rewrote something like Closure Compiler in C, I doubt you could get more than a 2x-4x speedup. The bigger fruit lay in minimizing the amount of work you do each time through the optimization loop.

The problem with most hobby projects that write transpilers is, people take a simple app like TodoMVC and say "look how amazing the edit/refresh is with this transpiler", but design decisions made early can come back to bite you much later when you're trying to handle a codebase with hundreds of thousands of lines.

Transpiled higher level languages tend to encourage people to create many layers of abstraction and re-use a ton of existing libraries, which tends to bloat the inputs to the compiler.

> a global optimizing compile tends to be dominated by the O(f(x)) of the optimization passes

Really good point. Closure Compiler, at least historically, rejected many valid optimizations because they aren't common enough to justify growing the rate of number of passes.

> If you rewrote something like Closure Compiler in C, I doubt you could get more than a 2x-4x speedup.

Closure Compiler is written in Java. I don't know exactly where the state of the art of JIT is right now, but I would bet C wouldn't be that much faster.

Why not allow the user to specify the level of optimization?

I'd be interested in seeing what a large typescript project would look like run through https://github.com/angular/tsickle.