It should also be pointed out that you cannot measure / quantify the performance of a programming language. The best you can do is measure the performance of a specific implementation of the language on specific platforms, running specific programs.
So when you ask about "the fastest functional language", what you are really asking about the best of the current implementations of the language(s).
@igouy's comment raises the point that there are other performance measures for language implementation; e.g. compilation time. But that doesn't change the fact that application program's run time is an (indirect) measure of a language's implementation, not a measure of the language itself.
Consider Java for example. Suppose I write a single-threaded benchmark using solely language features of classic (Java 1.0) Java. If I compile and run using JDK 1.0, I will get a poor performance ('cos JDK 1.0 didn't have a native code compiler). If I go from JDK 1.1 to ... JDK 1.7, I will most likely get progressively better results. But this is not due to changes to the Java language ... because my benchmark is using the same language subset. Rather the speedup is due to improvements in the compilers, the runtime system and / or the implementation of class libraries. These are all implementation issues.
The other point is that these implementation differences can be really significant (e.g. orders of magnitude) for the same language. So the fact that the best implementation for language X is faster than the best (or only) implementation of language Y doesn't necessarily tell you a lot about the language itself.