Manipulating the source code leads to invoking the Swift compiler to generate the information required to drive the rich editing experience. I have found SourceKit’s performance in complex projects, especially those where there are a lot of third-party libraries or frameworks you’re linking against, to be pretty disappointing. Especially when you’re working with many ObjC dependencies.

Like many of Apple’s developer tools, if you stick to the happy path, performance is great. Practically that means compiling your source into a single Swift module and keeping the number of modules low. If you are a “basic” iOS or macOS app with just a couple of 3p libraries things work great. Real world projects are drastically more messy...

It wasn't until Swift came out that autocomplete for ObjC in Xcode finally started to function at a useful level.

Autocomplete in Swift is still broken but I don't think it's an issue with the language itself (autocomplete is probably easier to implement in Swift given the nature of the language).