This bandwidth is only about 800-900 Mbits/sec , Only 10% of the bandwidth between physical nodes. How can I improve my bandwidth when using a VPNaas ?

Environmental information：

file :  /etc/neutron/neutron_vpnaas.conf
service_provider = VPN:strongswan:neutron_vpnaas.services.vpn.service_drivers.ipsec.IPsecVPNDriver:default

file: /etc/neutron/l3_agent.ini
vpn_device_driver = neutron_vpnaas.services.vpn.device_drivers.libreswan_ipsec.LibreSwanDriver

OpenStack Rocky ovs + vlan + Centos7
network-1      192.168.113.12
network-2      192.168.113.13
compute-1     192.168.113.10
compute-2     192.168.113.15
router-1         192.168.130.41           netns on  network-1  node
router-2         192.168.130.47           netns on  network-2  node
vm1               192.16.11.19              on compute-1
vm2               192.16.12.7                on compute-2

Between two physical network nodes：

 iperf -c 192.168.113.13 -t 600 -P 4     ### 
------------------------------------------------------------
Client connecting to 192.168.113.13, TCP port 5001
TCP window size:  459 KByte (default)
------------------------------------------------------------
[  4] local 192.168.113.12 port 8963 connected with 192.168.113.13 port 5001
[  5] local 192.168.113.12 port 8965 connected with 192.168.113.13 port 5001
[  3] local 192.168.113.12 port 8967 connected with 192.168.113.13 port 5001
[  6] local 192.168.113.12 port 8969 connected with 192.168.113.13 port 5001
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-600.0 sec   171 GBytes  2.44 Gbits/sec
[  3]  0.0-600.0 sec   171 GBytes  2.44 Gbits/sec
[  5]  0.0-600.0 sec   160 GBytes  2.29 Gbits/sec
[  6]  0.0-600.0 sec   160 GBytes  2.29 Gbits/sec
[SUM]  0.0-600.0 sec   661 GBytes  9.47 Gbits/sec

Between two physical compute nodes：

 iperf -c 192.168.113.15 -t 600 -P 4
------------------------------------------------------------
Client connecting to 192.168.113.15, TCP port 5001
TCP window size:  289 KByte (default)
------------------------------------------------------------
[  4] local 192.168.113.10 port 9582 connected with 192.168.113.15 port 5001
[  3] local 192.168.113.10 port 9584 connected with 192.168.113.15 port 5001
[  6] local 192.168.113.10 port 9586 connected with 192.168.113.15 port 5001
[  7] local 192.168.113.10 port 9588 connected with 192.168.113.15 port 5001
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-600.0 sec   142 GBytes  2.03 Gbits/sec 
[  6]  0.0-600.0 sec   155 GBytes  2.21 Gbits/sec
[  3]  0.0-600.0 sec   170 GBytes  2.43 Gbits/sec
[  7]  0.0-600.0 sec   195 GBytes  2.80 Gbits/sec
[SUM]  0.0-600.0 sec   661 GBytes  9.47 Gbits/sec

When I use Vpnaas plugin ( IPsec vpn ) connect two networks:

[root@vm-1 ~]# iperf -c 192.16.12.7 -t 60 -P 4
------------------------------------------------------------
Client connecting to 192.16.12.7, TCP port 5001
TCP window size:  170 KByte (default)
------------------------------------------------------------
[  6] local 192.16.11.19 port 50778 connected with 192.16.12.7 port 5001
[  4] local 192.16.11.19 port 50774 connected with 192.16.12.7 port 5001
[  3] local 192.16.11.19 port 50772 connected with ...