I experience the same thing. Odd part is that, shen just selecting everything in the canvas i get around 40% cpu usage.
But however, if I paste a screenshot (1280x994 pixels) that selection makes the system utilizy 80% CPU, maybe because of the "resize" functionality on that selection.
Anyway, this is on a dual core E6400 running 32-bit Vista.
the CPU-usage is insanely high for this activity.