Two offices connected using site-2-site VPN on Fortigate 40C devices. Both units were running flawlessly on Centurylink 6Mbps/768Kbps. Once we changed over to Spectrum 100Mbps/7Mbps the VPN is 1/10 of the previous performance or worse! The download speeds are excellent outside of the VPN on both sides. Charter Spectrum has looked at both sides. They claim everything is perfect. The remote office only connects to a Terminal Server using RDP sessions. If I connect outside of the VPN it is amazing! However the added security of the VPN is a must. So this was done as a trial and not acceptable. Fortinet has been working on this for months and continues to blame Charter. Over the VPN, the RDP sessions are very slow. If you attempt to copy a 200MB file over the VPN it will never get past calculating. If you attempt to copy a 2MB file it may or may not get there. I have recreated the VPN tunnel and had Fortinet verify things.
Here is what Fortinet says:
The TCP Retransmissions occur when the transmitting server does not receive TCP-ACK from the receiving end. Some possible causes of not receiving TCP-ACK are:
This might be due to bad cabling/interface or duplex mismatch on ISP.
I agree this does sound like a duplex mismatch type end result, but is it possible Charter doesn't know what they have?