On my machine (AMD-based win7/64) all 4 cores are 100% utilized (but not 100% of the time) when exporting.
Lr is known to have issues (with CPU utilization) on some systems. I know some people have managed to improve such performance, but I don't think there is any universal magic bullet to do so. I'm guessing the root of the problem, if not in the hardware itself, is in low-level system device drivers or BIOS, but now I'm getting pretty far out on a limb. Maybe somebody else will be more help.
Consider updating your BIOS and/or low-level device drivers. Also, try with hyper-threading disabled.
Sorry I can't be more help, but the problem you are having is far from universal, in case you were wondering...