It will make no difference at all as far as I can figure out. Even if it caused a small difference in throughput from the external drive to the data controller, CD writing software shouldn't be fazed at all by that, so I don't see any real need to worry about it. At the CD drive end of the chain, it's the buffer memory that handles things and that isn't going to be affected by slow throughput anyway, only the opposite.
I have to add that your external drive cannot be as fast as your internal drive unless your internal drive and/or its controller is/are very slow. If you're judging it by spin speed or something like that, don't be misled into thinking that any one figure fairly represents performance.