It is slow because the CD only has 16 bit processing. The time varies greatly from one machine to another based on the hardware components of the unit. On this high end computer, the 750 GB disk took five days. When I had only half the disk encrypted, it took about 2.5 days.
Please be aware that you must not stop or pause the CD decryption of your disk, or you will lose all of its data. When the disk gets down to having zero percent remaining encrypted, there is actually still nearly 1 % left to decrypt, so do not stop it until it clearly states the disk is decrypted.
For future reference, be aware that it is better to decrypt from within the operating system when you can: safer and faster.