Does not really require you to drop to a lower-level language for performance, unless you really need to use assembly for something.
OCaml is great for the application level stuff, and also performs quite well in general. But for building fundamental parts of the stack in which perf is important (like the multi-threading runtime, the garbage collector, etc.) you need to drop down to C++, C, or Rust.
OCaml is definitely much nicer to use than C and C++. But I don't find it that much nicer to use than Rust TBH.
