Is "performant" a valid word? What's the alternative?

If a lexical item composed of alphabetical characters consists of one or more morphemes and communicates a meaning which can be comprehended from the context without reference to a dictionary, then that lexical item is more of a word than most of the words in the dictionary.

Edit: Well, 'most' is stretching it, but you know what I mean.

As a back-formation from "performance" it does make some sense. On the other hand, the alternatives "fast" and "efficient" are much more common, at least in the circles I travel in.