“Why are CDR discs optimized for recording at different speeds? Why does it matter?”
The different speed limitations discs have are mostly due to the chemical properties of the disc and the dye that is used. Different dyes will heat and cool differently from the laser and some formulations simply cannot heat up or cool fast enough for very high speed recording. Recently the recorders/burners have begun to employ sophisticated power controls for the laser to better optimize it for the media being used.