Like Jeff Boulter, I believe "performant" is a legitimate word. How better to succinctly describe a system as performing to expectations? This is one of the few linguistic quirks from business software development that I've willingly incorporated into my own vocabulary. There it joins several others that I've involuntarily added, such as "impact" as a verb and "enterprise" as a synonym for business.
("Vocabularic", however, is not yet a word. Though it does return 12,300 Google results, which lends some populist legitimacy.)