Yeah, this is the big question : )
One of the big Swift sites, recently updated with V2 information, and he is really exited, and thinks we are close to writing Swift code with C++ performance.
I agree, that there are a lot of negative posts about Swift performance, and V1 certainly was a slow bugger.
The problem is to a large extend the ARC overhead, and this is obviously something Apple is working on.
The thing with Swift is, that you have to treat it differently. If you just go about business as in ObjC-C, you can run into problems. In Swift you should ex. replace classes with structs wherever possible, so CCColor will in the future be a mutable struct, and not as it is now, an immutable class which creates ARC overhead. Also, there are several techniques to lower this overhead, including a new feature named Module Wide Optimisation, which greatly reduce this.
So while I can not give you any final numbers at this point, I am much less worried than I was a month ago. And things will only improve from here. I am confident we will have good performance.