Optical disk drives slower (have larger access time) than magnetic disk drive due to following reason -
Sectors of an optical disk are on continuous spiral track. This data organization results in slower random access of a sector than in case of concentric tracks organization (used by magnetic disks) because in case of concentric tracks, data access mechanism always finds sectors on a given track at a fixed distance from the center.
In case of optical disks, each sector has the same length regardless of its position from the center. This data organization requires a more complicated drive mechanism because disk's rotation speed must vary inversely with the radius. That is, the drive must slow down disk's rotation speed to read/write sectors towards the periphery of the disk and speed it up to read/write sector towards the center of the disk. This access mechanism leads to slower data access (larger access time) as compared to magnetic disks in which case the disks rotate at a constant speed irrespective of the location of accessed data.
Since optical disk is a removable media, it is prone to scratches, dust, sticky prints (including fingerprint), etc. While handling. This causes the read mechanism to employ error correction mechanisms, like re-read, varying angular velocity, data reconstruction using parity, etc.
The entire read/write and disk assembly is not a sealed unit. Hence, the disk drive cannot spin the disk at fast speed as it damage the disk and other parts.
Access time of optical disks varies in the range of 100 to 300 milliseconds. Compare this with the access time of hard disks, which varies in the range of 10 to 30 milliseconds.