So you have an external CD drive that is connected to your Mac or another external HDD via Firewire?
Firewire 400 offers transfer speeds of up to 35MB/s, FW800 offers 65-75MB/s.
A CD drive will not read with such speeds, it will read from the inner sectors of a CD (or DVD) faster than from the outer sectors (the ones being farther away from the centre), as the CD drive has to slow down the rotational speed.
So all in all it comes down to the rotational speed offered by your CD drive, labeled as 2x, 4x, 24x, ....
8x drives offer a little more than 1MB/s, 16x drives almost 2.5MB/s.
So if you have a 72x drive (the fastest around at this time) you can get almost 10MB/s.