It’s good practice to create immutable collections in all cases where the collection doesn’t need to change. Doing so makes it easier for you to reason about your code and enables the Swift compiler to optimize the performance of the collections you create.

とか書いてあるけど、これ将来的に最適化される余地を残すために推奨(ありがち)とかじゃなくて、実際に何らかの最適化がされるのかな。