Help with troubleshooting performance issue

Hi guys
I hope you can help me shed some light on what's going on.
For my lab, I am trying to setup a PPtP server and a PPtP client.
I am running CentOS 6, and I downloaded the RPM package for the server.

The configuration is pretty straight forward, and the client can connect to
the server no problems using pptpsetup.

Performance, however, is bad.
The machines are connected directly to each other via Cat5e cable.

SCP and FTP transfer speed using the machines' direct IP addresses are
good, I am getting pretty close to the wire speed (around 40-50MBps, I am
not too fussed).

SCP and FTP transfer speed using the machine's VPN IP address are bad, I am
getting about 4 or 5MBps (1/10th) of the original transfer speed.

I grabbed another machine and put Ubuntu server on it, running pptp server,
still the same thing.

What I have done to check:
1. Tried lower the MTU on both connections manually, this seems to help a
bit but not much (speed fluctuates between 5-7MBps).
2. Tested another distro with another machine to rule out any NIC issue.
3. Swapped the network cables (not that I think the cable is playing up, I
was getting good speed on the cable).

Can you give me ideas to troubleshoot? Thanks.

------------------------------------------------------------------------------
Own the Future-Intel(R) Level Up Game Demo Contest 2013
Rise to greatness in Intel's independent game demo contest. Compete 
for recognition, cash, and the chance to get your game on Steam. 
$5K grand prize plus 10 genre and skill prizes. Submit your demo 
by 6/6/13. http://altfarm.mediaplex.com/ad/ck/12124-176961-30367-2

_______________________________________________
Poptop-server mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/poptop-server