How to speed up Compressor 4?

I'm running Compressor 4 on a 6-core MacPro, doing conversions that take quite a while, an hour or 2 each. However, the CPUs aren't any near completely busy, and the disk I/O is nowhere near capacity. So I tried asking it to do multiple conversions. However it queued them up, and only did one at a time.

Is there a setting I can tweak to get Compressor to process multiple files simultaneously to take advantage of all of the processing power my computer has? Alternatively, why is one conversion so slow when the CPU, memory use, and disk I/O are all just small fractions of what is possible?