--> nearly exactly as advertised, 64 kbit/s bidirectional. (Note: looks like you have a certain burst allowance for the first 10 seconds).
You should set smaller blocks for iperf, because otherwise you won't see anything in between:
~$ iperf3 -c speedtest.wtnet.de -p 5204 -t 86 --bidir -V -l 8K -b 64K -t 13 -O 2
iperf 3.20
Linux ubuntu 7.0.0-10-generic #10-Ubuntu SMP PREEMPT_DYNAMIC
Control connection MSS 1428
Connecting to host speedtest.wtnet.de, port 5204
TCP MSS: 1428 (default)
Target Bitrate: 64000
[ 5] local 2a01::::fe13:9208 port 57444 connected to 2a02:2028:ff00::f9:2 port 5204
[ 7] local 2a01::::fe13:9208 port 57448 connected to 2a02:2028:ff00::f9:2 port 5204
Starting Test: protocol: TCP, 1 streams, 8192 byte blocks, omitting 2 seconds, 13 second test, tos 0
[ ID][Role] Interval Transfer Bitrate Retr Cwnd
[ 5][TX-C] 0.00-1.00 sec 8.00 KBytes 65.5 Kbits/sec 6 2.79 KBytes (omitted)
[ 7][RX-C] 0.00-1.00 sec 8.00 KBytes 65.5 Kbits/sec (omitted)
[ 5][TX-C] 1.00-2.00 sec 8.00 KBytes 65.6 Kbits/sec 2 1.39 KBytes (omitted)
[ 7][RX-C] 1.00-2.00 sec 8.00 KBytes 65.6 Kbits/sec (omitted)
[ 5][TX-C] 0.00-1.00 sec 8.00 KBytes 65.5 Kbits/sec 4 2.79 KBytes
[ 7][RX-C] 0.00-1.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 1.00-2.00 sec 8.00 KBytes 65.5 Kbits/sec 2 1.39 KBytes
[ 7][RX-C] 1.00-2.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 2.00-3.00 sec 8.00 KBytes 65.5 Kbits/sec 4 2.79 KBytes
[ 7][RX-C] 2.00-3.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 3.00-4.00 sec 8.00 KBytes 65.5 Kbits/sec 3 1.39 KBytes
[ 7][RX-C] 3.00-4.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 4.00-5.00 sec 8.00 KBytes 65.5 Kbits/sec 1 1.39 KBytes
[ 7][RX-C] 4.00-5.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 5.00-6.00 sec 8.00 KBytes 65.5 Kbits/sec 5 1.39 KBytes
[ 7][RX-C] 5.00-6.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 6.00-7.00 sec 8.00 KBytes 65.5 Kbits/sec 1 1.39 KBytes
[ 7][RX-C] 6.00-7.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 7.00-8.00 sec 8.00 KBytes 65.5 Kbits/sec 3 1.39 KBytes
[ 7][RX-C] 7.00-8.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 8.00-9.00 sec 8.00 KBytes 65.5 Kbits/sec 1 1.39 KBytes
[ 7][RX-C] 8.00-9.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 9.00-10.00 sec 8.00 KBytes 65.5 Kbits/sec 5 1.39 KBytes
[ 7][RX-C] 9.00-10.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 10.00-11.00 sec 8.00 KBytes 65.5 Kbits/sec 2 1.39 KBytes
[ 7][RX-C] 10.00-11.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 11.00-12.00 sec 0.00 Bytes 0.00 bits/sec 2 1.39 KBytes
[ 7][RX-C] 11.00-12.00 sec 8.00 KBytes 65.5 Kbits/sec
[ 5][TX-C] 12.00-13.00 sec 0.00 Bytes 0.00 bits/sec 3 1.39 KBytes
[ 7][RX-C] 12.00-13.00 sec 8.00 KBytes 65.5 Kbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
Test Complete. Summary Results:
[ ID][Role] Interval Transfer Bitrate Retr
[ 5][TX-C] 0.00-13.00 sec 88.0 KBytes 55.4 Kbits/sec 36 sender
[ 5][TX-C] 0.00-13.05 sec 16.0 KBytes 10.0 Kbits/sec receiver
CPU Utilization: local/sender 0.1% (0.0%u/0.1%s), remote/receiver 0.0% (0.0%u/0.0%s)
CPU Utilization: local/receiver 0.1% (0.0%u/0.1%s), remote/sender 0.0% (0.0%u/0.0%s)
snd_tcp_congestion cubic
rcv_tcp_congestion htcp
[ 7][RX-C] 0.00-13.00 sec 104 KBytes 65.5 Kbits/sec 1 sender
[ 7][RX-C] 0.00-13.05 sec 104 KBytes 65.3 Kbits/sec receiver
snd_tcp_congestion htcp
rcv_tcp_congestion cubic
iperf Done.
Not that bad. Does seem not to matter wether you're on IPv4 or IPv6
~$ ping -q -i 0.06 -3 -c 500 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
--- 8.8.8.8 ping statistics ---
500 packets transmitted, 497 received, 0.6% packet loss, time 30280ms
rtt min/avg/max/mdev = 20.560/27.935/79.315/5.227 ms, pipe 2
ubuntu@ubuntu:~$ ping -q -i 0.06 -3 -c 500 dns.google
PING dns.google (2001:4860:4860::8844) 56 data bytes
--- dns.google ping statistics ---
500 packets transmitted, 497 received, 0.6% packet loss, time 30270ms
rtt min/avg/max/mdev = 20.470/28.874/140.598/9.328 ms, pipe 3