mtu of TUN is not set correctly on OS X
OS X client is using kernel-libipsec and tun device, but the TUN created by OS X client has a MTU of 1500, which is causing problems as websites like apple.com/speedtest.net has some problem with PMTU.
#define TUN_DEFAULT_MTU 1400
The default MTU should be 1400, but ifconfig clearly says
utun3: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1500 inet 10.22.0.1 --> 10.22.0.1 netmask 0xff000000
I suppose this is a bug in tun device implementation.