问如何快速减少++操作符替换后的while-循环的执行时间？

EN

var first = string1.startIndex
var last = first.advancedBy(n, limit: string1.endIndex)
var myarray = [String](count: string1.characters.count-n,
                       repeatedValue: "")

for i in 0..<myarray.count {
    myarray[i] = string1[first...last]

    first = first.advancedBy(1)
    last = first.advancedBy(n, limit: string1.endIndex)
}

var string1 = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Fusce pretium diam at arcu suscipit, sed molestie mi rhoncus. Donec neque sapien, luctus ultricies ex sed, tristique auctor sem. Praesent varius ullamcorper nisi a commodo. Mauris eu rutrum enim, sit amet hendrerit velit. Curabitur iaculis neque ut tellus pretium, at convallis odio posuere. Integer ultricies diam ex, sit amet faucibus odio ultrices eu. Morbi tincidunt felis tellus, eget gravida est suscipit quis. Nam nunc ipsum, molestie sit amet neque at, aliquam maximus lacus. Sed bibendum massa a lorem eleifend, non eleifend felis commodo. Vivamus eget sem luctus, aliquam dolor nec, finibus diam. Morbi id justo vehicula, eleifend arcu nec, hendrerit neque. Donec ex ante, fringilla id rhoncus nec, porta id tortor. Donec dapibus nisl nibh, non posuere lacus egestas non. Duis massa quam, elementum sit amet volutpat eget, tincidunt sodales urna. Donec at congue ligula, a sollicitudin tortor."
string1 = string1+string1+string1 // 2880 characters
let n = 4

var first = string1.startIndex
var last = first.advancedBy(n, limit: string1.endIndex)
var rangeArr = [Range<String.CharacterView.Index>](
        count: string1.characters.count-n,
        repeatedValue: first...last)

for i in 0..<rangeArr.count {
    rangeArr[i] = first...last

    first = first.advancedBy(1)
    last = first.advancedBy(n, limit: string1.endIndex)
}

var myarray = [String]()
for var i in string1.startIndex ... string1.endIndex {
    myarray.append(string1[i])
}