OK I found the cause of that weirdness. I had slightly changed my test code since I posted it here weeks ago. After undoing that change I'm seeing the same thing you do.

But this raises more questions, because what I changed is the test code generation. I'm now generating a million different strings instead of adding the same string a million times (note the \(i) at the end of the string):

    func generateTestData() -> [String] {
        var a = [String]()
        for i in 0..<N {
            a.append(",,abc, 123  ,x, , more more more,\u{A0}and yet more, \(i)")
        }
        return a
    }

The running time of generateTestData() isn't what we measure but apparently the performance improvement you found only works if the same string is used every time. Otherwise performance drops.