Why iOS 27 slows down even the new iPhone 18 Pro at first

All iPhones are slower than you'd expect when they are new or when you've updated iOS, and Indexing is generally the culprit. Here's exactly what's happening and what you can do about it.
While indexing is continuing, you'll see "Optimising Search and Siri" in Settings.
Not to drag this out, the short answer is that all you absolutely have to do to speed up your iPhone is wait. You can help it along a bit, but the iPhone is indexing all of the data you've got on it and when that's done, it won't