Even 6x to 8x is still less than 2MB/s - should be no problem to read from scsi or RAM to CD writer on the IDE bus. If you're trying to write an image from a hard drive on the IDE bus this could be a major problem though.
If the image file is small enough, and you have enough RAM, try moving the image file to your RAM disk and writing to disk from there. Otherwise you'll have to find a way of forcing MakeCD to write at a lower speed. I'm sure there must be a setting somewhere which allows this?
Is the hard drive containing your image file on the same IDE cable as your CD writer? I'm not sure whether it might be better to have them on seperate IDE lines, or whether it would make little difference.