Optimizing adding dashes to a long Swift String

I am trying to take a hex string and insert dashes between every other character (e.g. "b201a968" to "b2-01-a9-68"). I have found several ways to do it, but the problem is my string is fairly large (8066 characters) and the fastest I can get it to work it still takes several seconds. These are the ways I have tried and how long they are taking. Can anyone help me optimize this function?

//42.68 seconds
    func reformatDebugString(string: String) -> String
    {
        var myString = string
        var index = 2
        while(true){
            myString.insert("-", at: myString.index(myString.startIndex, offsetBy: index))
            index += 3
            if(index >= myString.characters.count){
                break
            }
        }

        return myString
    }

//21.65 seconds
    func reformatDebugString3(string: String) -> String
    {
        var myString = ""
        let length = string.characters.count
        var first = true
        for i in 0...length-1{
            let index = string.index(myString.startIndex, offsetBy: i)
            let c = string[index]

            myString += "\(c)"
            if(!first){
                myString += "-"
            }
            first = !first
        }

        return myString
    }

//11.37 seconds
    func reformatDebugString(string: String) -> String
    {
        var myString = string
        var index = myString.characters.count - 2
        while(true){
            myString.insert("-", at: myString.index(myString.startIndex, offsetBy: index))
            index -= 2
            if(index == 0){
                break
            }
        }

        return myString
    }