Need advice analyzing bad throughput with vpn

Hi, I'm trying SSL VPN and get really bad throughput (about 1-2 Mbit/s). Normal untunneled traffic reaches about 60-70 Mbit/s. Here is my pcap data:

https://www.cloudshark.org/captures/8226ca16543d

I am connecting from a client (192.168.1.15) to a server (192.168.1.10) through a gateway. I'm aware of the general problems with SSL-VPN as compared to e.g. L2TP or PPTP (ie specifically, running TCP over TCP), but I hope someone can give some feedback on whether my pcap data contains anything obvious. I can see there are fast retransmissions but I'm not sure if they would contribute to the slow throughput as they are not that many. There are quite a few Dup ACK but same thing here - I don't know if these can explain the slow speed. What about MTU? But I don't know what should be the right value here.

Am I looking at the wrong things? What should I be looking at?

Any comments or ideas are greatly appreciated!