Last month I finished writing a lot of the "Performance" chapter for our ASP.NET book. I used the word performant quite a bit, and was a little surprised when the editorial review told me that performant isn't a word.
I didn't believe it, but after some checking I was forced to agree:
It's not in the dictionary: http://www.m-w.com/dictionary/performant
It's not in the Google definitions: http://www.google.com/search?q=define%3Aperformant
It's not even in Wikipedia: http://en.wikipedia.org/wiki/Performant
Looks like we need to let the scholars know - there are 18,300 uses of the non-word "performant" in scholarly papers: http://scholar.google.com/scholar?q=performant