That is true of languages, but it is in fact the case that there are algorithms that run in O(n) time when written impurely, but require O(nlogn) or such when written in a purely functional style, even in state of the art FP languages. Since FP languages provide impure facilities for use in the real world, you can still use the O(n) algorithms, but at the cost of purity.

So the relevant question here really is not "are functional languages inherently slow?", but "is pure functional programming inherently slow?". AFAIK, the answer to that question is not conclusively proven yet.

It's not so simple. Any language (that is actually implemented) is always designed with regular specific consideration for implementation strategies. In fact I would go so far as to say the implementation is the design in practice.

For example choice of boxing or unboxing values is an integral part of language design and has profound impact on the implementation's performance both for speed and memory usage.

That may be technically true, but in practice some language features require more overhead than others.

If a language requires a feature where making it "fast" is so difficult that nobody can implement a fast version of it, then I think it's fair to say that language is slower than the alternatives.

Knowing that language X can, in theory, be faster than language Y does me no good at all if there are no implementations of language X that are actually faster than Y.

Excellent point - a formal system can't have 'speed.'