This is old, but it came up in my search.
I thought I would answer, to help anyone else that comes along.
Technically speaking, any resource that is pegged during processing, is considered a bottleneck, and restricting performance.
In this case, pegging the CPU is desirable, because this means our system is operating at its maximum capacity.
I'm running a workstation with an ASUS MB, 20 cores and 128gb of RAM. GAEA pegs every core for me.
If you are not seeing 100% CPU Utilization, then your bottleneck is somewhere else.
Check your disk utilization and see if you are pegging that. If so, that is the reason your cores having very little to do.
The bottleneck could also be in the I/O channels on your processor and MB. Your MB might be too busy processing requests from different systems, to serve up data to your CPUs in a timely manner. For instrance heavy usage of USB 3.0 ports.