HIGH CPU with VXLAN OVER IPSEC

Hi GUYS

One of my customer implemented vxlan over ipsec with 2 fortigate 1500D firmware version 5.6.0

When we inject the traffic in the  normal tunnel everything seems to be fine including the performance however when sending through vxlan we can see that  we of the core cpu is using 50% and the other cpu are ok

Can someone tell me if this is normal behavior and how can we fix this issue

Thanks

james