It’s a word. Mostly used in the software world.
It’s somewhat trendy, but it’s not crazy to think about, either.
If you don’t like it, you’re not alone.
I use it here to mean this:
WordPress running at its absolute best, with the best plugin choices, the best theme choice, on the best server installation, with the least amount of third-party support (CDNs).
In other words, a WordPress installation that is strong and powerful and relevant to that company’s needs.
If there is code that is unused, but slowing down that organization’s use of WordPress, regardless of whether it is in core, than that installation of WordPress is not performant.
Plus, it’s kind of catchy: Performant Press. Wouldn’t you want your WordPress to be more performant than it is?
Yes, you would.
And yes, you can have it.