That's always the answer: "the compiler has gotten better and will get better still". Your claim was that Objective-C has all this "extra work" and indirection, but Swift actually has more places where this applies, and pretends it does not. With Objective-C, what you see is what you get, the performance model is transparent and hackable. With Swift, the performance model is almost completely opaque and not really hackable.
>None of the above is possible in Objective-C, though, because of its type system.
What does the "type system" have to do with any of this? It is trivial to create, for example, extremely fast collections of primitive types with value semantics and without all this machinery. A little extra effort, but better and predictable performance. If you want it more generically, even NeXTSTep 2.x had NXStorage, which allowed you to create contiguous collections of arbitrary structs.
Oh...people seem to forget the Objective-C has structs. And unlike Swift structs they are predictable. Oh, and if you really want to get fancy you can implement poor-man's generics by creating a header with a "type variable" and including that in your .m file with the "type variable" #defined. Not sure I recommend it, but it is possible.
The fact the Foundation removed these helpful kinds of classes like NXStorage and wanted to pretend Objective-C is a pure OOPL is a faulty decision by the library creators, not a limitation of Objective-C. And that Foundation was gutted by CoreFoundation, making everything even slower still was also a purely political project.
In general, you seem to be using "Objective-C" in this pure OOPL sense of "Objective-C without the C" (which is kind of weird because that is what Swift is supposed to be, according to the propaganda). Objective-C is a hybrid language consisting of C and a messaging layer on top. You write your components in C and connect them up using dynamic messaging. And even that layer is fairly trivial to optimize with IMP-caching, object-caching and retain/release elision.
Chapter 9 goes into a lot of details on Swifft: https://www.amazon.com/gp/product/0321842847/
A few Swift issues surprised me, to be honest. For example native Swift dictionaries with primitive types (should be a slam dunk with value types and generics) are significantly slower than NSDictionary from Objective-C, which isn't exactly a high performance dictionary implementation. About 1.8x with optimizations, 3.5x without.
This is another point. The gap between Swift and Objective-C widens a lot with unoptimized code. Sometimes comically so, 10x isn't unusual and I've seen 100x and 1000x. This of course means that optimized Swift code is a dance on the volcano. Since optimizations aren't guaranteed and there are no diagnostics, your code can turn into a lead balloon at any time.
And of course debug builds in Xcode are compiled with optimization off. That means for some code either (a) the unoptimized build will be unusable or (b) all those optimizations actually don't matter. See "The Death of Optimizing Compilers" by D.J. Bernstein.
Anyway, you asked for some links (without providing any yourself):
"Several seconds to parse 1.5MB JSON files"
But really, all you need to do is run some real-world code.
You also mention looking at the assembly output of the Swift compiler to tune your program. This alone should be an indication that either (a) you work on the Swift compiler team or (b) you are having to expend a lot more effort on getting your Swift code to perform than you should. Or both.