I disabled the VPN NAT Rules, the VPN interface and the VPN itself. After this the speed test results varied from 250 to 300Mbps. Looks like the VPN has something to do with the slow speeds, even the sites that weren't using the VPN.
It occurred to me that I recently updated from 2.3.4 to 2.3.4-1. I might not have noticed the slow speeds until this latest mess with comcast. If the slow speed was only on the VPN I might think it is a problem with the VPN server, but that seems unlikely given the effect on non-VPN sites.
Could something have happened with the VPN client during the upgrade?
If so, can I download or update the VPN client?