You’re actually mostly correct with your party line assessment. Your ISP puts all its users through various sets of processing equipment. To keep their costs down, they minimize the number of channels they use. The more users per channel, the slower things get because you’re all sharing the bandwidth available on the channel you’re on. It looks from your data, that between 11 AM and 12:30 pm, things slow down a lot. You should call your ISP and tell them what’s happening. It may take more than one phone call. Be polite but persistent. They should have the ability to move your account to a different channel - they can balance the user throughput between different channels of their gear.
It it looks like you’re testing using the TestMy Dallas server. Is it the one closest to where you are?