Centos vpn server speed problems where to dig?

Question

VPN

An xl2tpd server is up on centos6. 2 offices are connected to it. The first office has a router that automatically raises the vpn channel and redirects any incoming connections addressed to it to one ip in its network (dvr).
Recently, when trying to get video from the registrar, a problem came to light - the picture is transmitted with the wildest brakes. I decided to check the vpn connection speed for this, temporarily redirected all incoming connections in the first office to a test win7 machine, launched iperf on it in server mode, connected to it with iperf from the machine from the second office - the speed was about 10Mbits, but when I tried to upload a file to ball speed is limited to 300KB/sec.
What could be the problem? The MTU on all ppp interfaces of the vpn server is set the same - 1500. But I don't understand what the problem is.

Answer 1

1) which transport is tcp or udp.
2) ping different packet sizes, which gives.
3) what is the speed from each point to external resources?
4) perhaps someone overloaded one of the channels, turn off other clients for the duration of the test.
5) measure the speed in both directions.
At first glance, it looks like a large mtu, you can try to reduce it.