windows CPU is at 100%, why is my CPU frequency down?

windows CPU is at 100%, why is my CPU frequency down?

I'm looking at Resmon on Windows 8 Basic. I have chosed the "high
performance" power plan, which I thought would keep my CPU at the highest
levels possible.
When the machine is (mostly) idle, the CPU Frequency is at 95%+, and the
CPU usage is under 10%.
When I put the system under heavy load, the CPU frequency starts jumping
around between 10% and 110%. The CPU usage jumps around as an inverse of
the CPU frequency (which makes sense).
Why would my CPU frequence DECREASE when the system is under load? Is this
some kind of thermal management issue (the CPU is getting hot, so the
system slows the CPU to allow heat to dissipate)?