Open Radar

 
From Apple:

There are no plans to address this based on the following:

You can use CTLineEnumerateCaretOffsets() instead.

//
//  main.swift
//  CTChoke
//
//  Created by Colin Rofls on 2017-04-18.
//  Copyright © 2017 Colin Rofls. All rights reserved.
//

import Foundation

func makeString(withLength len: Int) -> String {
    return String(
        bytes:(0..<len).map({ _ in (uint8(arc4random() % 25) + 65) }),
        encoding: String.Encoding.utf8)!
}

func timeClosure(title: String, closure: ()->()) {
    let startTime = CFAbsoluteTimeGetCurrent()
    closure()
    print("\(title) took \(CFAbsoluteTimeGetCurrent() - startTime)")
    
}

for length in [024, 1024*16, 1024*32, 1024*64, 1024*128, 1024*256, 1024*512, 1024*1024] {
    let attrString = NSAttributedString(string: makeString(withLength: length))
    let ctLine = CTLineCreateWithAttributedString(attrString)
    timeClosure(title: "\(length) bytes", closure: {
        let midpoint = length / 2
        let pos = CTLineGetOffsetForStringIndex(ctLine, midpoint, nil)
    })
}

> 24 bytes took 0.000228047370910645
> 16384 bytes took 0.212086021900177
> 32768 bytes took 0.810679018497467
> 65536 bytes took 3.23262000083923
> 131072 bytes took 13.2235760092735
> 62144 bytes took 59.6346539855003