It's an aesthetic argument. If you make your constraint tight enough, you're forced into concocting a new word to fix some imagined failing of the existing language. We don't need an adjective that signifies performance and we also don't need an adjective that signifies performance that starts with "perform".

Instead, you can rearrange the wording a little to convey the exact same meaning. By working within the constraints, an elegant solution emerges. By all means add words if they are clever or pleasing or convey a nuance that can't be achieved with what already exists. Performant and functionality don't meet those criteria.