Fastest way to read specific line from string

    //public static string ReadLine(string text, int lineNumber)
    //{
    //    var reader = new StringReader(text);
//
    //    string line;
    //    int currentLineNumber = 0;
//
    //    do
    //    {
    //        currentLineNumber += 1;
    //        line = reader.ReadLine();
    //    }
    //    while (line != null && currentLineNumber < lineNumber);
//
    //    return (currentLineNumber == lineNumber) ? line :
    //                                               string.Empty;
    //}

    private string[] splitString = null;
    public static string prevText = "";
    public static string ReadLine(string text, int lineNumber)
    {

        if (!text.Equals(prevText)) Instance.splitString = text.Split('\n');
        
        prevText = text;
        return Instance.splitString[lineNumber - 1];
    }