Bonus graph! (Y axis is speed in mbps.)
Of course Comcast is throttling traffic. This shouldn't surprise anyone, and I at least am not convinced by Comcast's attempts at an explanation. It's simply too easy to get away with it. Think about it. ISPs aren't actually held to any sort of standard with respect to the speeds they advertise, the speeds you supposedly pay for, and the speeds they actually deliver. Oh, you're only getting 3 Mbps down on your 15 Mbps line? Must be those pesky "network conditions." Oh, you're only getting 200 kbps while using Netflix but your connection is fine otherwise? Since your traffic is being routed through 5-10 devices/nodes, it clearly must be something wrong with one of those systems, not ours. Look! I'm testing your connection right now and it says it's operating normally! There's nothing else I can do!
Give me a fucking break. Fuck all of these companies. I'm generally a pretty calm guy that doesn't wish I'll will on anyone, but I really do hope the people responsible for these issues have bad things happen to them. It's such a flagrant and opportunistic abuse of the situation for no reason other than greed. Unlike others (including the OP) I'm not convinced that we're headed down some draconian path of content packages for the internet. There are too many entities - from the average Joe like you and me to many multibillion dollar companies - that have an interest in seeing that not happen that I don't think it will. For the time being, though, it's miserable. We're getting scammed, we all know we're getting scammed, and a combination of apathy and malfesience keeps things the way they are.