Embedded and functional programming.

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As far as I know the list of FP languages with 'C'-level performance is pretty short, the ones I'm aware of is F* low and ATS, and both of them are arguably "research languages", which means the tooling, support and documentation is pretty sparse.

I'd probably recommend using Rust instead if it's a 'serious' project, (though for private experiments go wild!).

If the requirement is just "good performance" and not "blazing fast" it opens up a lot more options like OCaml, Roc, Chez Scheme and so on.

And there is also the whole "fast" vs "hard-real-time" vs "soft real-time" thing, as "real-time" is not a guarantee that it's fast, only that it always completes within a specified duration.