> It happened enough times that I'm suspicious the ISP had some way to detect if you run a speedtest, and then prioritized traffic to that customer.

ISPs definitely know when you run a speedtest.net test. 90% of the time, the data for that comes from boxes/services they host themselves. It’s not exactly hidden either. It’s a typical program any ISP can sign up for and you can easily see the destination the test is running against. I won’t be surprised if some have some logic to prioritize particular subscribers plan once they have detected a test from them. They probably view it as a “customer support calls reduction” feature.