I'm using GCD / DispatchQueue. have a look guys who ever is stuck on this.

func loadLevel() { var cluesString = "" // Holds the clues var solutionString = "" // Holds the number of letter that a solution has to be var letterBits = [String]() // Holds all letter bits

    DispatchQueue.global(qos: .background).async { [weak self] in
        if let levelFileURL = Bundle.main.url(forResource: "level\(self!.level)", withExtension: "txt") {
        if let levelContents = try? String(contentsOf: levelFileURL) { // creates "level1.txt" string to load the file from the App Bundle
            var lines = levelContents.components(separatedBy: "\n")
            lines.shuffle() // each time the game starts will mix up the letter groups

            for (index, line) in lines.enumerated() { // return the item and its index
                let parts = line.components(separatedBy: ": ")
                let answer = parts[0]
                let clue = parts[1]

                cluesString += "\(index + 1).  \(clue)\n"

                let solutionWord = answer.replacingOccurrences(of: "|", with: "")
                solutionString += "\(solutionWord.count) letters\n"
                self?.solutions.append(solutionWord)

                let bits = answer.components(separatedBy: "|")
                letterBits += bits
            }
        }
    }

    DispatchQueue.main.async { [weak self] in
        self?.cluesLabel.text = cluesString.trimmingCharacters(in: .whitespacesAndNewlines)
        self?.answersLabel.text = solutionString.trimmingCharacters(in: .whitespacesAndNewlines)

    letterBits.shuffle()

        if letterBits.count == self?.letterButtons.count {
            for i in 0 ..< self!.letterButtons.count {
                self?.letterButtons[i].setTitle(letterBits[i], for: .normal)
            }
         }
      }
   }
}

}