The fact that 3DES is slower is actually an advantage if you are trying to prevent against brute-force attacks. Given that we are encrypting binary data, it's unlikely that knowledge of the plaintext (for example, knowing that it's probably US-ASCII and so the 8th bit is probably always zero) is not going to be available to the attacker. Other potential attacks, such as related plaintext attacks, will also not be available to the attacker. Given that brute force attacks are the most likely threat scenario, using a 3DES which is slower is a reasonable choice.