Help with SBS Infrastructure -- Remote Sites

Hello Everyone,

I am in a situation where I am supporting a site comprising of approximately 20 users. I have recently brought another site online using a site-to-site IPSEC VPN built using pfSense. I've got it all running perfectly, however there is 1 issue; Computers at the remote sites are PAINFULLY slow to log in. At this point there is only one Windows XP machine at the remote site. When logging in, it takes a 3-5+ minutes between the time you enter your password and press enter. Then it gets stuck on the "Applying Computer Settings" for another 2-3 minutes.

This is making for a very negative impact for the users (as I can imagine).

Both the branch office and the 'head office' (where the SBS 2008 is) are connected via ADSL (meaning 1 meg upload at both ends), and unfortunately due to reasons I won't go into now I am unable to upgrade these internet links. My first thought to troubleshooting this was that there was insufficient bandwidth between the sites. However, after extensive monitoring of the IPSEC link between the two sites, I see that it is almost idle at all times.

How can I rectify this issue? I want to avoid purchasing another server and making it a DC at the branch office site if possible.

Any insights appreciated.