That's true. Mono is generally slower than the CLR, though it has been improving ever since Xamarin took over. There's also an option now to use LLVM as the JIT

That's true. Mono is generally slower than the CLR, though it has been improving ever since Xamarin took over. There's also an option now to use LLVM as the JIT compilation engine in Mono; for server-based applications (where steady-state performance matters more than start-up time), this can greatly improve the performance thanks to LLVM's optimizer. With the improvements in the latest 3.2.x branch, I think Mono's performance is at least "good enough" for many use cases. FWIW, I do a bit of F# + Mono development on FreeBSD 9 (and now FreeBSD 10) and have found the performance to be quite good. I wrote an installation guide (http://fsharp.org/use/freebsd http://fsharp.org/use/freebsd) if you want to give it a try, e.g., in a VirtualBox VM.