It doesn't mean anything.

What do you mean when you describe something as "performant"? Do you mean that it's fast, efficient, powerful, flexible, or that it handles big tasks? "Performant" gets used to mean any of those things, when there are perfectly good, specific, and strong words to do the job.

It weakens your message.

By taking a verb, "perform", and making it into an adjective, "performant", qualitative statements which used to be active become passive. What sounds more exciting to you, "It's performant" or "It's blazing fast"?

Jargon turns people off.

Using the internal language of a sub-group — jargon — is one way people try to signal they're part of that group. If you're talking to other engineers, they don't need the signaling. If you're not talking to engineers, jargon is adding a subtext of "I'm in, you're not" to your message. Do you really want to do that?

Finally, it's not in the dictionary.

"But words get in to the dictionary by being used!" Yes, and that's another good reason to avoid jargon and meaningless buzzwords.