TCP performance for OpenVPN vs delay

[LovingFox]

I have got some issue in TCP performance depended on delay.

My lab schema:

CPE --- (p1) Router (p2) --- Server​

The CPE connects to the Server by OpenVPN using TCP. I know about TCP-over-TCP but TCP proto is mandatory! Next I do series of network benchmarks through the OpenVPN tunnel.

Series:

1. WITHOUT artificial delay.
2. WITH artificial delay of 30 ms (RTT) made on a Router's p1 and p2.
3. WITH artificial delay of 60 ms.

So the series resaults:

1. "to Server": 40 mbps, "from Server": 50 mbps
2. "to Server": 34 mbps, "from Server": 38 mbps
3. "to Server": 20 mbps, "from Server": 10 mbps

Rate limit in test 1 due to CPU on CPE. It's normal. But why third test with delay 60ms reduces performance "from Server" multiple 5 times while "to Server" only 2 times?

P.S. CPE is an OpenWRT on a Banana PI R2 board. Server is a Ubuntu server, Router is a Ubuntu server too. All devices connect via Ethernet. CPE intefrace is 100M. Others — 1G.

TCP congestion control for both (CPE and Server) is BBR:

• net.ipv4.tcp_congestion_control = bbr
• net.core.default_qdisc = fq

Benchmark commands on a CPE:

• "to Server": iperf3 -c ovpn_server_ip
• "from Server": iperf3 -c ovpn_server_ip -R

Commands for artificial delay on a Router:

test 2:

• p1: sudo tc qdisc add dev ens224 root handle 1: netem delay 15ms
• p2: sudo tc qdisc add dev ens256 root handle 1: netem delay 15ms

test 3:

• p1: sudo tc qdisc add dev ens224 root handle 1: netem delay 30ms
• p2: sudo tc qdisc add dev ens256 root handle 1: netem delay 30ms