What do you think the difference is between "performant" and "fast"?

"fast" is in my dictionary.

Thanks for pointing out that "performant" is not in any english dictionary! (http://weblogs.asp.net/jongalloway/performant-isn-t-a-word) Didn't know that.

Besides that, your "fast" code will not be as fast as it can be if you are only relying on purely functional code, at least on current architectures.

Do be fair, he said "impure language" and not "impure code".

"Looks like we need to let the scholars know - there are 18,300 uses of the non-word "performant" in scholarly papers" <- If that's not sarcasm, something is wrong. Performant is a word!

It is a word, just not an English one it seems! It certainly is a German word. That might explain a lot ;-) It might also explain the many occurrences, as there are many Germans writing papers in English.