Надо улучшить скорость выполнения кода (формирование длинной строки)

public static string RandomWord(int Number) 
{
/*  RandomWord causing it returns a random word consisting 
    of letters specified in the call, for example RandomWord(56); 
    return the word consisting of 56 random letters. */
            
    Random RandomIndex = new Random(); //Create an object RandomIndex, we will serve it to pull out a random array      ArrayLetter letter.
            string[] ArrayLetter = {"q", "w", "e", "r", "t", "y", "u", "i", "o", "p", "a", "s", "d", "f", "g", 
                    "h", "j", "k", "l", "z", "x", "c", "v", "b", "n", "m"}; //Array hammered ArrayLetter letters on the two layouts.
            
    string OutputValue = null; //OutputName used to store ready-made speech, but until it is empty.
            for (int i = 0; i < Number; i++)
            {
            /*  In a series of random letters pulled out of the array and stored in ArrayLetter OutputName 
            as many times as was specified when calling RandomWord.*/
 
                string ElementArray = ArrayLetter[RandomIndex.Next(0, 25)]; //In a random letter written ElementArray fished array ArrayLetter.
                OutputValue += ElementArray; //In the words of OutputName obtained by adding the letters received from the variable ElementArray.
            }
            return OutputValue; //No comments.
}

public static void Main(string[] args)
{
    Console.WriteLine(RandomWord(99999));
    Console.ReadLine();
}

int u = Environment.TickCount;
            Random r = new Random();
            int Number=9999999;
            StringBuilder sb =new  StringBuilder(Number);
            for (int i = 0; i < Number; i++)
            {
                sb.Append((char)r.Next(97, 122));
                
            }
            
            this.Text= (Environment.TickCount-u).ToString();
            MessageBox.Show(sb.ToString());

namespace Trening_589
{
    class Program
    {
        public static void Main(string[] args)
        {
            for (int i = 0; i < 10; i++)
            {
                System.Console.WriteLine(RandomWord(10));
            }
            System.Console.ReadLine();
        }
        
        public static string RandomWord(int Number)
        {
            System.Random Rand = new System.Random();
            System.Text.StringBuilder StringBuilder = new System.Text.StringBuilder(Number);
            for (int i = 0; i < Number; i++)
                StringBuilder.Append((char)Rand.Next(97, 122));
            return(StringBuilder.ToString());
        }
    }
}

System.Random Rand = new System.Random((int) DateTime.Now.Ticks & 0x0000FFFF)

System.Random Rand = new System.Random();

  public static void Main(string[] args)
    {
        for (int i = 0; i < 10; i++)
        {
 
            System.Console.WriteLine(RandomWord(10)); 
            
        }
        System.Console.ReadLine();
    }
    static System.Random Rand = new System.Random((int)DateTime.Now.Ticks & 0x0000FFFF);
    public static string RandomWord(int Number)
    {
 
        System.Text.StringBuilder StringBuilder = new System.Text.StringBuilder(Number); 
        for (int i = 0; i < Number; i++)
            StringBuilder.Append((char)Rand.Next(97, 122));
        return (StringBuilder.ToString());
    }