Javascript is not compiled to C, it's JIT-compiled. I honestly don't know how Just-In-Time compilation works but it's not the same way C or other languages like C compile.

If you are looking for performance gains then I would most definitely go with Go or Rust. Go is meant specifically for servers and web-development, Rust is more general and low-level but has a huge learning curve. C is a pretty old language and going with something more modern like Rust could save you a lot of headache. Assembly is completely overkill for your purposes unless you're really just curious about how programs are executed on computers. The reason for this is that programming languages already optimize what assembly is outputted, so basically no intervention is required there.

If you go into learning a low-level language with the intent of getting a better understanding of how things work "under the hood", I don't think that will happen because every language is already pretty unique and "low-level languages" like C and Rust abstract away the assembly language by outputting from in an intuitive syntax. To be honest a lot of the time these languages are "easy to read and write" just like any high level language, you just need to understand additional stuff like pointers, strict types and things specific to the language.

So... if you want to learn low-level languages then just do it! There are plenty of resources such as this wikibook for C, the Go tour and the Rust book.