Linux doesnt have compute mode and doesnt need it.  Most likely this is because of the cpu and chipset on the board.  Try swapping it. with a rig that gives full hash.  If so then its a lack of PCIe lanes as this is what slows down Zombie mode.  You need a full 16 pcie lanes to get the full speed.  So google the cpu for how many pcie lanes and then look in mobo manual and see what else uses and shares lanes.

I have a few boards that cant provide the full lanes and others that can.  Those with 16x to the slot achieve full speed the others do not.