Poor xkcd. Frowning upon people freely interchanging their, they're and there is not being grammar police (unlike complaining about split infinitives, dangling preposition etc). It seems such people that make this mistake are too lazy and arrogant to correct themselves and are too eager to call names. The placard ruined what would have been an average comic..
By using "performant" in the title (which is known to set off the language nazis), the author is practically begging for the discussion to devolve into this tangent.
It's imprecise: What is it even supposed to mean? Because it's a made-up word, who knows for sure? Does it mean "better performing?" If so, why not just say that? It's not that many more keystrokes. Better performing in what way? CPU? Resource utilization? Say so. You've put a lot of thought and effort into writing something, why blow it by using an imprecise pseudo-word? The author is undermining his own credibility, telling us "I don't care enough about the topic to even pick an actual word, let alone summarize, in more detail, what I mean to discuss."
My understanding is that performant is totally legal French, meaning efficient or effective, with usage dating back at least 4 decades. If you're not into stealing random words from other languages, I have to question why you're into English in the first place.
