There are 2 possibilities
Location of Data:
CDs are read at a constant linear rate. This means that at the read head the CD must travel over it at a constant speed. Data on a CD has the same size per bit on the inside and outside, meaning that there is more data per revolution on the outside of the disc. Therefore, to provide a constant speed of data input over the reading head, the disc spins slower when reading the outside.
Because of this, if the game needs to constantly read data that is near the outside of the disk, it will spin slower
This would explain different rotational speed for a single disk
Read Speed:
CDs and DVDs are read at different speeds
For the Playstation2 these are the specs:
24x speed CD-ROM [3.6 MB/s], 4x speed DVD-ROM [5.28 MB/s]
24x speed CD-ROM can typically spin at 5k to 12k rpm
4x speed DVD can typically spin at 2.2k to 6k rpm