Decoding custom packets with Python using Twisted

This server require fast decoding of packets from clients, as I am expecting 5000 concurrent clients that is sending packets possibly every second. This is a game server (5000 players walking send packets each n every step).

I want to see if my implementation is fast enough or there are still room for improvements. The encryption isn't on this code as I only want to tackle the decryption for now.

The decode function is taken from a JavaScript implementation, This is the best output I got after trying to translate it. (though the translation is correct because the results are same from JavaScript and now Python)

def dataReceived(self, data):
    packet = decode(data)
    (counter, packet_id, username, password, unknown) = unpack('<B H 16s 16s 16s', packet)
    print counter
    print packet_id
    print username
    print password
    print unknown

For example the above implementation is pretty straight forward but the concern here is the decode(data) part.

I am new to Python and Twisted so I had to do some maybe absurd way on implementing this.

This is my decode function:

def decode(packet_data):
    cl = 0x6e
    internal_rounds = 2
    i = 0

    seqX = bytearray.fromhex('HEXSTRINGREMOVED')
    seqY = bytearray.fromhex('HEXSTRINGREMOVED')
    result = []

    while i < (len(packet_data) - 4):
        dl = ord(packet_data[i])
        bl = seqX[(i * 2) + 1]
        al = seqX[i * 2]
        bl2 = seqY[(i * 2) + 1]
        dl2 = seqY[i * 2]
        j = internal_rounds - 1

        while j > 0:
            if bl <= dl:
                dl -= bl
            else:
                bl = (~bl) & 0xFF
                bl += 1
                dl += bl

            dl ^= bl2
            j -= 1
        if al <= dl:
            dl -= al
        else:
            al = (~al) & 0xFF
            al += 1
            dl += al
        dl ^= dl2
        dl ^= cl
        result.append(dl)
        cl ^= dl
        i += 1

    return "".join(map(chr, result))

I did this dl = ord(packet_data[i]) because I keep getting int -= char error.

Also did this return "".join(map(chr, result)) because I kept getting errors with unpack because the return was previously return result which result is an array.

So what do you think? Should I also have used unpack_from instead? But I don't know how to make a buffer in Python. Is the data received already a buffer?