The magnitude of the spike in CPU utilization is a metering artifact of the IPMI interface.
The IPMI system management driver may consume a lot of CPU, but it does this with low priority, so it should not block other CPU users. It uses much less CPU on a busy system. It basically soaks up what would otherwise be idle time.
modprobe $IPMI_SI_MODULE_NAME > /dev/null 2>&1
modprobe $IPMI_SI_MODULE_NAME si_force_kipmid=1 > /dev/null 2>&1 # disable kipmi0