High CPU usage when idle as KVM guest

I am trying to upgrade OpenWrt 15.05 x86 to LEDE 17.01.4 on my KVM host, running Proxmox VE 4.4 on Intel J1900 processor. The new LEDE costs 3% of 4C or 12% of 1C when idle on both x86 and x86_64. When I started the VM when Openwrt, the CPU idle reduced to 0.5% of 4C.
The Openwrt has Linux 3.18.20, and LEDE has 4.4.92.