Comment by skarap

`ip link set qlen 1000 dev tun0` (so it matches the eth0 queue, instead of being an order of magnitude smaller) on both ends of the tunnel, and you get 90% of direct connection speed from openvpn:

  $ iperf3 -c 10.1.0.2
  Connecting to host 10.1.0.2, port 5201
  [  4] local 10.1.0.1 port 38274 connected to 10.1.0.2 port 5201
  [ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
  [  4]   0.00-1.00   sec  3.12 MBytes  26.1 Mbits/sec    0    802 KBytes
  [  4]   1.00-2.00   sec  22.3 MBytes   187 Mbits/sec    0   2.24 MBytes
  [  4]   2.00-3.00   sec  21.4 MBytes   179 Mbits/sec    0   2.45 MBytes
  [  4]   3.00-4.00   sec  23.3 MBytes   196 Mbits/sec    0   2.39 MBytes
  [  4]   4.00-5.00   sec  23.4 MBytes   196 Mbits/sec    0   2.58 MBytes
  [  4]   5.00-6.00   sec  21.5 MBytes   180 Mbits/sec    0   2.70 MBytes
  [  4]   6.00-7.00   sec  23.5 MBytes   197 Mbits/sec    0   2.68 MBytes
  [  4]   7.00-8.00   sec  22.6 MBytes   189 Mbits/sec    0   2.78 MBytes
  [  4]   8.00-9.00   sec  22.4 MBytes   188 Mbits/sec    0   2.45 MBytes
  [  4]   9.00-10.00  sec  23.5 MBytes   197 Mbits/sec    0   2.69 MBytes
  - - - - - - - - - - - - - - - - - - - - - - - - -
  [ ID] Interval           Transfer     Bandwidth       Retr
  [  4]   0.00-10.00  sec   207 MBytes   174 Mbits/sec    0             sender
  [  4]   0.00-10.00  sec   207 MBytes   174 Mbits/sec                  receiver

  iperf Done.