Openvswitch high cpu usage

Dear community members,

I've installed the latest version of x86-64 v18.06.1 on my device for testing purposes and there is high cpu usage ( please read rest ) with this specs:

CPU J1900 4 core 2.0 Ghz
4 Port Lan
4GB Ram

Components installed on OpenWRT:
Openvswitch v2.8.2

OVS Config:

root@OpenWrt:~# ovs-vsctl show
    Bridge switch
        Port "eth3"
            Interface "eth3"
        Port switch
            Interface switch
                type: internal
        Port "eth2"
            Interface "eth2"
    ovs_version: "2.8.2"

added bridge called "switch"
eth2 and eth3 added to bridge "switch"
STP enabled on "switch"
eth2 to pc1
eth3 to pc2
pc1 IP:
pc2 IP:
(no vlan no gw, just pure layer 2 networking )

test utility "iperf3"
test mode: run iperf server on pc1, run iperf client on pc2
iperf config on server(pc1): iperf -s
iperf config on client(pc2): iperf -t 60 -c
test result: avg 850mbps transfer rate
here the CPU usage:

Is it usual?

I've tested all above configuration with the same device but with ubuntu 14.04 ( not OpenWRT ) as OS,
test result: avg 930mbps transfer rate
CPU usage: 2~3 percent

more BW transfer rate and low CPU usage!

Is there any misconfiguration on my settings with OpenWRT?