Investigate MTU performance tweaks

[kylemanna]

Review OpenVPN's Gigabit Network Wiki Page and consider implementing some performance tweaks.

Currently, ~ 150 Mb/s consumes 100% of a Digital Ocean 1 CPU / 512 MB droplet.

A larger MTU should improve encryption/decryption efficiency and IPC between kernel and user space daemon.

Test Case

Client

~ ❯❯❯ curl http://cachefly.cachefly.net/100mb.test http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  http://cachefly.cachefly.net/100mb.test  > /dev/null
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  100M  100  100M    0     0  12.2M      0  0:00:08  0:00:08 --:--:-- 13.5M
100  100M  100  100M    0     0  14.5M      0  0:00:06  0:00:06 --:--:-- 14.4M
100  100M  100  100M    0     0  14.4M      0  0:00:06  0:00:06 --:--:-- 13.9M
100  100M  100  100M    0     0  14.8M      0  0:00:06  0:00:06 --:--:-- 15.0M
100  100M  100  100M    0     0  14.1M      0  0:00:07  0:00:07 --:--:-- 13.9M
100  100M  100  100M    0     0  14.4M      0  0:00:06  0:00:06 --:--:-- 14.8M
100  100M  100  100M    0     0  15.5M      0  0:00:06  0:00:06 --:--:-- 15.7M
100  100M  100  100M    0     0  14.3M      0  0:00:06  0:00:06 --:--:-- 14.1M
100  100M  100  100M    0     0  14.4M      0  0:00:06  0:00:06 --:--:-- 14.8M
100  100M  100  100M    0     0  14.9M      0  0:00:06  0:00:06 --:--:-- 15.0M
100  100M  100  100M    0     0  15.4M      0  0:00:06  0:00:06 --:--:-- 15.5M

Result

• Server: (running kylemanna/openvpn:0.2)
• Client: (running 2012 rMBP + TunnelBlick with OpenVPN 2.3.4)
• User 😞

Suggested by @bydavy