A few points:
We rewrote a lot of C/C++ HPC code in Swift, we easily match C performance in most instances. In some cases faster. There is no general performance cost with Swift, in general you can match other compiled systems languages. Of course there are a lot of optimization opportunities, but current performance can be quite good.
Comparisons to Javas GC seems silly. Every time I see Java idioms in Swift, even in our own code, it raises red flags. It's often an indication that it should be redesigned. I don't think many (any?) are looking for Swift to take on the side-effects of tracing GC. If you recall, Apple deprecated GC over a decade ago (determinacy & RAM do matter). That said Swift ARC is could use some serious investment to improve performance.
@Joe_Groff - I'd like to see immortal object marks - its seems like a relatively simple directive to avoid retain-release cycles in a good deal of circumstances.