"Performant" != "Fast". A performant thing is something which performs well, by whatever metrics are applicable to that thing. A performant algorithm is one that has both low time and space complexity. A performant web browser is one that renders the page quickly while also implementing the most web standards. A performant mobile device is one that is fast while drawing a minimal amount of power so that its battery lasts and it doesn't overheat. So "performant" is a very performant word, as it is a single word that means exactly what it needs to mean.