Unexpected CPU utilization on LTM

You would expect 12 cores to operate better. Are they on the same hypervisor with the same networks etc? You can get 'pinning' where there is a limited number of IP addresses and source ports. You would notice that because one tmm would go high utilisation but others would not be affected. Assuming that everything else is the same then i'd look at platform config ie memory assigned, resource provisioning, licensing etc and also at the LTM config to check there isn't logging, iRules etc that may make a difference.