I've got Fusion Broadband from Sonic through AT&T. Today, interactive sessions were a bit laggy, and the Sonic speedtest is much slower than my line speed. Speed test in the AT&T modem portal shows full speed, basically unchanged from when it was installed 5 months ago. Using beta.speedtest.net to an AT&T test server shows full speed. Using beta.speedtest.net to the Sonic San Jose server (which is my physically nearest server) shows the slow download speed again. Upload speeds are normal. Only download is slow, about 5% of full speed.
I am not using a VPN and all of my LAN traffic goes through my local, separate firewall box. I can see the speed tests in traffic graphs on the firewall, and there is basically no traffic other than the speed tests (so no high traffic malware, botnets, peer to peer, network backups, dropbox, onedrive, etc. running on the network).
It looks like slowness in the network between ATT broadband and Sonic at San Jose.