Ok, the symptoms occurred, I unplugged AC power and top immediately showed the CPU usage return to normal for Xorg/kwin. Within a couple of minutes (I was running a CPU intensive task), it happened again, I yanked the power and saw the same immediate improvement.
So, it seems you are on to something! I'm guessing this is something to-do with when the system has to adjust fan/clock rates in response to heat? Note that I am running with CPUFREQ=no on the boot line. nvidia-settings shows the GPU temperature as ~78 degrees with maximum performance being used in Powermizer with clock rates 530/799 MHz for the GPU/memory clock respectively.
I'm hoping there is a fix or tweak for this since flicking the power continuously is a nice workaround but a little inconvenient!