> performant is not considered a real word in English, although I commonly see it used...
This is close to being a contradiction in terms. The purpose of words is to communicate, and if a word is being successfully used to communicate -- which clearly it is -- what exactly does it mean to say that isn't "considered a real word"?
Also, "Considered" by whom? The dictionary? Dictionaries are descriptivist -- they record usage, they don't prescribe it [0]. If it isn't in the dictionary yet, that's only because the usage hasn't been used widely and/or for long enough that it meets the inclusion criteria [1].
(As it happens, "performant" is at the stage where it is starting to pass those thresholds, and is now in the OED [2])
[0] http://englishplus.com/news/news1100.htm
[1] http://public.oed.com/about/frequently-asked-questions/#qual...