Well it depends on the machine. But I have gotten very acceptable results on a crappy Acer n310 on relatively non trivial projects - a scripting language interpreter for example.

Does anyone happen to know how efficient F# is on mobile devices using the .NET Compact Framework compared to C#? Thanks.

Wrote an "Hello, world" form for CF on my Samsung Blackjack, just for kicks. Can't speak to runtime performance, but the startup time was on the slow side. I suspect this was due to having to load the F# runtime. Did not try to compile in "--standalone' mode, which could help this a lot.

I've been doing some development on the Xbox360 using the XNA framework, which is based on the compact framework.
When it comes to performance, the major issue is to keep garbage collection under control. As F# may have a tendency to generate more short-lived objects (closures, tuples), F# may actually be worse than C# if you don't pay attention to this.

I haven't actually do the test but I believe F# should have nearly the same perfomance as C# as its using the same CLR.