Throttling at the "signalling level" as you suggest seems to be a pretty good guess going by the graph that I attached. This is typical of what I get. I just wish they would throttle at what I'm paying for. It's like buying something that costs $9.50 total, giving them $10.00 and not getting my change back.
I have only recently discovered TestMy.net. It's a fantastic service that's being provided and has explained my irritation with Centurylink over the years. It is only recently that the service has become stable where I live (west central Missouri) so I guess they're doing something to improve with the exception of providing faster speeds. Every Mbps counts in a rural setting.
RMcQ got a reaction from Fuckyouiway in How do ISP's set your download speeds?