No where in the article does it mention that they had performance issues. Some developers write in JavaScript and it's fast enough. Swift performance is more than acceptable for most apps.
Sure, but it is slower and more maintainable. So "gain maintainability at a cost to performance" is correct. Which is generally what you want in most domains, but in areas where performance is more critical than maintainability you would rather use C still.
I don't know how Javascript plays into that - I assume Javascript is slower than Swift, but I don't think it is more maintainable.
We're not choosing religions here. Language is a technical decision (with business impact due to hiring availability). You choose the language that fits your goals best, not which one has a better saint.
"You choose the language that fits your goals best."
That's often an abused statement with no actual significance. If you are writing native iOS apps, you basically get two, maybe three, choices.
There is always a group of developers who will rush ahead and there's a group that will lag behind. It has more to do with human nature than the technical features of a language. That's why a year ago I started collecting a bunch of Swift resources in one place. Ive got almost 1500 urls.
Most blogs/articles on iOS development are now in Swift. If you are learning iOS 9, for example, it's easier to go with Swift. Swift is now the path of least resistance.
I seriously doubt most apps have more than a few performance critical areas that need to be in C -- and Swift in no way prevents you from jumping out to an Objective-C wrapper class for these exceptions.