I've just been monitoring my cpu usage in openoffice calc when cells have been copied vs when they haven't and seen a dramatic increase in cpu usage for the Xorg process. The additional rendering required is a box with scrolling dashed lines around the cells that have been copied. The issue stands regardless of whether the window is minimised. Obviously it takes /some/ cpu power to render, but to increase an i7 by a more or less constant 7% usage seems slightly overkill. If anything surely this should impact on the gpu?
I've found that turning off "Anti-Aliasing" speeds up OpenOffice on my system. The setting is in Tools->Options, OpenOffice.org->View. You might also want to experiment with turning on and off Hardware Acceleration.