Question Improving speed in my code

[Galactus]

Is there any way to improve the speed in my script??
Thanks in advance, appreciate any help.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace MegaTool
{
    public static class mega
    {

        private static readonly Rijndael RijndaelCbc;
        private static readonly byte[] DefaultIv = new byte[16];

        static mega()
        {
            RijndaelCbc = Rijndael.Create();
            RijndaelCbc.Padding = PaddingMode.None;
            RijndaelCbc.Mode = CipherMode.CBC;
        }

        public static byte[] ToBytes(this string data)
        {
            return Encoding.UTF8.GetBytes(data);
        }

        public static byte[] PrepareKey(byte[] data)
        {
            byte[] pkey = new byte[] { 0x93, 0xC4, 0x67, 0xE3, 0x7D, 0xB0, 0xC7, 0xA4, 0xD1, 0xBE, 0x3F, 0x81, 0x01, 0x52, 0xCB, 0x56 };

            for (int it = 0; it < 65536; it++)
            {
                for (int idx = 0; idx < data.Length; idx += 16)
                {
                    byte[] key = data.CopySubArray(16, idx);

                    pkey = EncryptAes(pkey, key);
                }
            }

            return pkey;
        }

        public static byte[] EncryptAes(byte[] data, byte[] key)
        {
            using (ICryptoTransform encryptor = RijndaelCbc.CreateEncryptor(key, DefaultIv))
            {
                return encryptor.TransformFinalBlock(data, 0, data.Length);
            }
        }

        public static T[] CopySubArray<T>(this T[] source, int length, int offset = 0)
        {
            T[] result = new T[length];
            while (--length >= 0)
            {
                if (source.Length > offset + length)
                {
                    result[length] = source[offset + length];
                }
            }

            return result;
        }

        public static string GenerateHash(string email, byte[] passwordAesKey)
        {
            byte[] emailBytes = email.ToBytes();
            byte[] hash = new byte[16];

            for (int i = 0; i < emailBytes.Length; i++)
            {
                hash[i % 16] ^= emailBytes[i];
            }

            for (int it = 0; it < 16384; it++)
            {
                hash = EncryptAes(hash, passwordAesKey);
            }

            byte[] result = new byte[8];
            Array.Copy(hash, 0, result, 0, 4);
            Array.Copy(hash, 8, result, 4, 4);

            return result.ToBase64();
        }

        public static string ToBase64(this byte[] data)
        {
            StringBuilder sb = new StringBuilder();
            sb.Append(Convert.ToBase64String(data));
            sb.Replace('+', '-');
            sb.Replace('/', '_');
            sb.Replace("=", string.Empty);

            return sb.ToString();
        }

    }
}