The "speed" that is "too slow" that you are referring to is the read speed I believe.

In order for the CD to burn, the CD burner needs to be able to be given the information being burned at the speed it is burning at. And that means your harddrive needs to be able to read fast enough to transfer the information to the burner. That being said, chances are that your harddrive is too slow to transfer fast enough. Try putting the data you wish to burn on a harddrive with a faster transfer rate.