The speed at which an optical drive runs denotes how much information can be transferred to and from the disk. Playing a DVD movie requires a 1x DVD which is 1.32MB/s, so a 8x DVD drive can transfer information at 8 x 1.32 = 10.56MB/s. Blue-Ray has a 1x of 4.5MB/s, so a 4x BD drive will be able to transfer information at 18MB/s.