ER-X - 1.9.0 throughput peaks at 40MB/s

I have an ER-X in a home environment.  My network is laid out as follows:

eth0 - wan

eth1~4 - switched lan

if I host an iperf3 server on the lan and use the built in bandwidth tool to test it, I get around 200mbps.  If I increase the TCP window to 1024, I can get to 300mbps.  The throughput is very stable, but capped far below what I would expect.  Others report speeds of 600mbps, even 800mbps+ on the ER-X, so I would expect a lot better.  

I have hwnat on, and whether deep packet inspection is enabled or disabled makes no difference.  Can anyone figure out what is wrong here?

My config:

firewall {
    all-ping enable
    broadcast-ping disable
    ipv6-receive-redirects disable
    ipv6-src-route disable
    ip-src-route disable
    log-martians enable
    name Inside-to-Outside {
        default-action drop
        description "LAN to WAN"
        rule 1 {
            action accept
            description All
            destination {
                address 0.0.0.0/0
                group {
                }
            }
            log disable
            protocol all
            source {
                address 172.16.1.1/24
                group {
                }
            }
        }
    }
    name Outside-to-Inside {
        default-action drop
        description out-in
        rule 1 {
            action accept
            description Established
            destination {
                address 172.16.1.0/24
            }
            log disable
            protocol all
            source {
                address 0.0.0.0/24
            }
            state {
                established enable
                invalid disable
                new disable
                related enable
            }
        }
    }
    receive-redirects disable
    send-redirects enable
    source-validation disable
    syn-cookies enable
}
interfaces {
    ethernet eth0 {
        address dhcp
        duplex auto
        firewall {
            out {
                name Inside-to-Outside
            }
        }
        speed auto
    }
    ethernet eth1 {
        duplex auto
        speed auto
    }
    ethernet eth2 {
        duplex auto
        speed auto
    }
    ethernet eth3 {
        duplex auto
        speed auto
    }
    ethernet eth4 {
        duplex auto
        speed auto
    }
    loopback lo {
    }
    switch switch0 {
        address 172.16.1.1/24
        mtu 1500
        switch-port {
            interface eth1 {
            }
            interface eth2 {
            }
            interface eth3 {
            }
            interface eth4 {
            }
            vlan-aware disable
        }
    }
}
port-forward {
    auto-firewall enable
    hairpin-nat enable
    lan-interface switch0
    rule 1 {
        description Optivault_80
        forward-to {
            address 172.16.1.101
            port 80
        }
        original-port 80
        protocol tcp_udp
    }
    rule 2 {
        description Optivault_443
        forward-to {
            address 172.16.1.101
            port 443
        }
        original-port 443
        protocol tcp_udp
    }
    rule 3 {
        description Optivault_32400
        forward-to {
            address 172.16.1.101
            port 32400
        }
        original-port 32400
        protocol tcp_udp
    }
    wan-interface eth0
}
service {
    dhcp-server {
        disabled false
        hostfile-update disable
        shared-network-name LAN-DHCP {
            authoritative disable
            subnet 172.16.1.0/24 {
                default-router 172.16.1.1
                dns-server 8.8.8.8
                dns-server 8.8.4.4
                lease 86400
                start 172.16.1.100 {
                    stop 172.16.1.200
                }
                static-mapping BD-S7e {
                    ip-address 172.16.1.111
                    mac-address a4:08:ea:15:7e:e0
                }
                static-mapping BD-ShieldTV {
                    ip-address 172.16.1.102
                    mac-address 00:04:4b:53:a6:11
                }
                static-mapping BD-Surface {
                    ip-address 172.16.1.110
                    mac-address c0:33:5e:17:2c:bf
                }
                static-mapping OptiVault {
                    ip-address 172.16.1.101
                    mac-address 00:11:32:50:83:5d
                }
                static-mapping UAP-AC-Pro {
                    ip-address 172.16.1.100
                    mac-address 44:d9:e7:f9:7b:b2
                }
                unifi-controller 172.16.1.101
            }
        }
        use-dnsmasq disable
    }
    gui {
        http-port 80
        https-port 443
        listen-address 172.16.1.1
        older-ciphers enable
    }
    nat {
        rule 5000 {
            description "LAN~WAN masquerade"
            destination {
            }
            log disable
            outbound-interface eth0
            protocol all
            source {
                address 172.16.1.0/24
            }
            type masquerade
        }
    }
    ssh {
        listen-address 172.16.1.1
        port 22
        protocol-version v2
    }
}
system {
    host-name ubnt
    login {
        [snip]
    }
    name-server 8.8.8.8
    name-server 8.8.4.4
    ntp {
        server 0.ubnt.pool.ntp.org {
        }
        server 1.ubnt.pool.ntp.org {
        }
        server 2.ubnt.pool.ntp.org {
        }
        server 3.ubnt.pool.ntp.org {
        }
    }
    syslog {
        global {
            facility all {
                level notice
            }
            facility protocols {
                level debug
            }
        }
    }
    time-zone UTC
    traffic-analysis {
        dpi enable
        export enable
    }
}


/* Warning: Do not remove the following line. */
/* === vyatta-config-version: "config-management@1:conntrack@1:cron@1:dhcp-relay@1:dhcp-server@4:firewall@5:ipsec@5:nat@3:qos@1:quagga@2:system@4:ubnt-pptp@1:ubnt-util@1:vrrp@1:webgui@1:webproxy@1:zone-policy@1" === */
/* Release version: v1.9.0.4901118.160804.1131 */