Best functionnal language?

I have a basic knowledge of how imperative languages work and I'd like to try out a functionnal language. I've heard they are actually slower than imperative languages.
Is that true? If it is, what advantages do those languages bring and what's the most relevant fp language right now?

in fact, von neumann's architecture can express any conceptions, so that OOP and FP is most publicized special case of pointers to functions