High CPU load when downloading

I am running the OpenWrt 21.02.3 on a Proxmox server as QEMU Standard PC (Q35 + ICH9, 2009) and the WAN Protocol is PPPoE.

When the download traffic is over 1 MiB/s (my max is 3.3 MiB/s) the CPU become hot and the FAN are spinning at max rpm.
When i am transferring files at 100 MiB/s between VM and Laptop CPU is at idle condition, why this is happening? Does it have to do with PPPoE protocol?

CPU when the traffic is idle

CPU when downloading at 3 MiB/s

Here the Proxmox settings of VM