Performant is not an adjective. Performantly is not an adverb. Niether is a word. Use real words when you write. Maybe write, "It didn't work properly / perform adequately," instead of writing nonsense.
One of my (subjective) requirements is that the word not be annoying (i.e. incongruent with the rest of the language). "Performantly" certainly was not, I think smalltalk is only complaining for complaining's sake.
> Niether is a word. Use real words when you write.
Aside from the glaring spelling error, this betrays a complete linguistic ignorance coupled with utter historical illiteracy. In short: Every new word, or semantic shift applied to an existing word, was railed against, often by people with no knowledge of linguistics, and many have since become productive parts of our language down to the present day. Some haven't, admittedly, but not because of the people who ranted against them.
For example, is 'television' a word? Can 'nice' be used to mean 'pleasant'? Can 'actor' be applied to a woman? Can a house made of wood ever become 'dilapidated'? (Seriously.)
If you're seriously opposed to 'non-words', the answer to all of those questions is NO.