From the DIGITAL FAQ web site:
The x speeds.
The x in a burning speed means almost nothing. It is not a multiplication of anything, especially when Z-CLV and P-CAV burning is involved. As you surpassed 4x speeds, time savings diminishes quickly, especially between 8x-16x speeds. Speeds range on several factors, including disc type, burning method, and the drive itself.
1x DVD burn takes approximately 55-60 minutes
2x DVD burn takes approximately 25-30 minutes
2.4x DVD burn takes approximately 20-25 minutes
4x DVD burn takes approximately 14-16 minutes
6x DVD burn takes approximately 9-12 minutes
8x DVD burn takes approximately 8-9 minutes
12x DVD burn takes approximately 7-8 minutes
16x DVD burn takes approximately 6-7 minutes
It's pretty much impossible, at least with current hardware, to spin a disc at 16x from start to finish. Blame physics. With 4x and slower media, the drive spun up to the burn speed, and burned 4x from beginning to end, using the CLV (constant linear velocity) burning method. The disc also had a uniform look on the burned dye.
With the advent of 8x media, Z-CLV (zonal constant linear velocity) and P-CAV (partial constant angular velocity) were introduced. Z-CLV starts at a speed like 4x, then at a certain point in the media, jumps to 6x, then 8x, etc., until it reaches the maximum speed. Sometimes a 16x Z-CLV burn will only burn a few hundred MB at 16x, which is why x speeds mean almost nothing anymore. P-CAV is similar, but does not jump speed. It increases velocity from 4x to 4.5x to 5x, etc., until it reaches it's top speed. Much like Z-CLV, it may not hit max speed until the last 30 seconds worth of burning. This is why a 12x burn is almost an identical wait time to the speed of a 16x burn.
Yeah, the x speed ratings are mostly a marketing ploy I guess.