[SPDK] 128k sequential read / write (fio) performance with spdk+vpp is not as good as the one with "kernel TCP"

Hello, 
I'm working on SPDK library + VPP, because some report said that VPP reduces the overhead of network. 
When I test with VPP (with mlx5 poll mode driver) and null device with spdk, 4k performance with VPP is much better than the default(kTCP).
But, 128k write performance with VPP is 30 percent lower than the one with kTCP. 
The configuration when VPP starts is as below.
Is there anyone why 128k write performance with VPP is not good as kernel ? 

I increase num-rx-desc and there is improvment, but it still lower than kernel. 
Thanks in advance. 

unix {
        nodaemon
        cli-listen /run/vpp/cli.sock

        # cli-listen localhost:5002

          full-coredump


}
session {
        evt_qs_memfd_seg
}
dpdk {
        dev 0000:03:00.0
        {
        }

}
socksvr {
#       socket-name /tmp/vpp-api.sock
        socket-name /run/vpp-api.sock
}
plugins {
        plugin default { disable }
        plugin dpdk_plugin.so { enable }
}