i just downloaded the file and im running a quad core 4gb ram and it STILL lags on my pc. And its a reasonably low poly character.

The problem lies with your rig itself i think. I got a warning telling me;

// Warning: Cycle on 'joint8_parentConstraint1.target[0].targetParentMatrix' may not evaluate as expected. (Use 'cycleCheck -e off' to disable this warning.)

So i searched for joint 8 and it appears you got something weird going on with the characters back. Perhaps try and re-rig it