I'd second F# and also in the same vein recommend Ocaml, in particular, if fast-running native code is a requirement. I haven't benchmarked it enough but I suspect that in most cases Ocaml might be a good deal faster.
I'm also very sympathetic to the non-functional aspects. I think being able to write imperative code easily (and it's surprisingly pleasant in ocaml/f#) is a huge plus.
As always, take these numbers with a grain of salt, but benchmarks seem to suggest that F# on .NET Core is significantly faster than OCaml [1]. I wouldn't be surprised if that were the case myself given how much work Microsoft has put into optimizing .NET Core for performance.
I'm also very sympathetic to the non-functional aspects. I think being able to write imperative code easily (and it's surprisingly pleasant in ocaml/f#) is a huge plus.