Hey Sarge,
I'm slightly buzzed (it's holiday season), but I'll try my best...
Quick ref:
VBR (Variable Bit Rate), is encoded so that sections that do not have a lot of information (read: quiet, or perhaps monotone), will use less space. Sections that have more dynamic action happening, will use more space.
So, the encoder will place a priority on the sections with more dynamic range, and encode them at a higher bit rate, and decrease the bit rate of the sections that do not need it.
Now:
When you decode that info, if it is CBR, the program already knows what to do with each frame of data to decode. If it is VBR, you have to analyze each frame as you do it.
Basically, it comes down to how much work the decoder has to do!
If you can do the same thing with 2 steps, as opposed to 10 steps... Get my drift?
VBR makes the computer work harder by an exponential amount. Not recommended for DJ work, IMHO.