Comcast does not participate in Netflix's OpenConnect project. If Comcast wanted to, they could receive a 4U server from Netflix (multiple ones, in fact, for each densely populated area they service) completely free of cost. Obvious costs for power and rack space apply, although the appliance itself is free.
This would then allow your Netflix viewing to go to a 4U server at a Comcast datacenter or colocation in the area instead of having the entire South East US try to fit through a small pipe at Marietta, GA.
Comcast's stubbornness to participate in the OpenConnect project is the reason for slowness, not throttling. Of course it's going to be faster over a VPN because whatever endpoint you're connecting to most likely takes a different AS path to Netflix.