The speed at which your computer's optical drive burns a disc can vary depending on what type of media you're using. For example, your PowerBook G4 computer's SuperDrive may have a maximum 8x DVD burn speed, but if the drive can't determine whether a disc is able to handle that speed, it will burn at the drive's minimum speed for the media, to ensure that the burn is successful.
When you insert a blank disc, it's not actually completely blank. Recordable and rewritable CDs and DVDs are manufactured with a small amount of written "pregroove" data that contains various disc attributes, including the disc type, part number, manufacturer, the kind of dye used, its writable capacity, rated write speed, and other items. This data resides in a very small area on the disc that is not part of the user-writable area.
The optical drive reads this data to determine how it should handle the disc. If the the optical drive doesn't recognize the media type as being burnable at its maximum speed, the drive speed will default to its slowest setting. This protects the burn reliability so you don't end up with unreadable discs that are only useful as drink coasters.
In addition, be aware that maximum burn speeds are reached toward the outer portions of the disc, which hold only a fraction of the disc's total writable area. Since data is written from the inside to the outer edge of the disc, the drive will not reach the maximum burn speed if you burn a disc with only a small amount of data because the written area doesn't approach the outer edge of the disc.
Note: Apple DVD-R media is manufactured to exacting specifications, and has been tested to burn at maximum rated speeds.
For more information about the factors involved in optical drive burning speeds, see "About optical disc drive burning and write speeds."