This word is one of my pet peeves as well. "Performant" is an ignorant and foolish way to say "high performance". It's like saying "voltant" instead of "high voltage". I'm all for coining new words that mean new things, but when someone says "performant" I treat it as a clue that they might be a sloppy thinker.
I don't like "performant" as a word, but I also recognize that is an irrational, emotional reaction on my part. I think it's valid to create an adjective that basically means "a thing that has high or acceptable performance". I see no reason to conclude people who use it are sloppy thinkers.
No, it's more like the wealth of other adjectives that follow the pattern of "having/with -ance" -> "-ant". Important, significant, brilliant, vigilant, cognizant, dissonant, exuberant, invariant, radiant, elegant, abundant, ...
I suppose you could say that someone using just about any of those words is trying to sound smart or overly formal, and so we should deny their status as words to bring the speakers down a notch.
"Performance" sounds like sloppy thinking too. What kind of performance are we talking about? Latency, throughput, false positive rate, false negative rate, click through rate, conversion to sales rate? There are a lot of axes on which to measure performance.
