In the first one, you are simply telling it to map and filter every element. Since you can do that to each element in an independent way and the code is more abstract, the map and filter can be done in parallel without you knowing (I'm not sure if would execute in parallel in Swift, I know that in java you can change the way it does it by using parallel streams).

But in the second example, you are telling it to iterate over each element, sequentially. Since you are telling step by step what needs to be done, it's not parallelizable unless you implement it to be so.