To my mind, the difference is that “performant” delivers results quickly, whereas “efficient” uses little energy. A performant solution might be efficient, but not necessarily, and vice versa. Does anyone else share this understanding or am I living in a linguistic bubble?
Not sure about Swedish (Daniel is Swedish), but in Finnish, we have the word “suorituskykyinen”, meaning “efficient” or “able to perform”. I can see how Finnish writers might want to use “performant” to replace this commonly used word. Perhaps there's a similar history for Swedish speakers? I sure have seen “performant” a lot in academic texts written by Finnish and Swedish speakers in the last 20 years.