Beautiful design- but it’s effectively dead on arrival due to iOS being too bloated for dated hardware, and it’s only going to get slower as newer iPhones get faster.
One of iOS 12's advertised features was that it's significantly faster than 11 on older hardware. And that seems to be true.
Performance is perfectly fine. Keep in mind the latest iOS release is actually faster than the previous. Phones have gotten to the same point as modern computers where they don’t need to be continually upgraded.