”Transpiles to c” - how do you generally optimize single line performance hotspots in that case?

I don't see why that would make any difference? The generated C code is just a build artifact in this case, similar to IRs often used by compilers internally. You don't think about e.g. gcc IR when you optimize hotspots in code written in C, though - you just look at the C source and the generated asm code. If you have, say, Zig transpiling to C (which it can do with `-ofmt=c`), you'd similarly look at the Zig source and the generated asm code.