I have posted a similar question to this in the past, but got little response. Perhaps some one out there has ran into this issue.
You're corporate network is dual-homed to the internet to two separate ISP's. You have several remote branch offices connected to your corporate site via point-to-point full T-1 links. The T-1's to the branch offices are perfectly fine and no where near being over-utilized. The protocol over the WAN is EIGRP. In addition to your ASA firewall (your corporate default-gateway) you have a separate Microsoft ISA server that is connected both internally and externally. Users in your corporation use this Microsoft ISA server to proxy with. Now the weird issue, users at and only at the remote branch offices frequently report that internet browsing is slow. However when you remove using the ISA server as a proxy the issue clears up and web browsing is fast. Again this only seems to be happening for the branch offices. Logic would point to the issue being with perhaps the ISA server...right? But since this happens with only the remote branch offices it's a bit strange. Any suggestions would be greatly appreciated.