It's vague. How does it "perform well"? Does it mean "faster"? "smaller"? Why not use a word that's not only more specific and well-known, but faster to read and pronounce?
FWIW, to me performant has connotations of conformant, so performant implies "performing to spec". I also think it implies in general doing what it's supposed to. Maybe that impression is incorrect but it is my impression.
