Some time a couple years ago or so, during a period of apparent network congestion, I ran a few speed tests using various services. Results from the popular service Ookla were always above the nominal speed of my service. Results from any lesser known service and my own ad hoc test were all significantly lower.
It's not proof my ISP was prioritizing the well known speed test's traffic to gaslight subscribers about the low network speeds, but I find it suspicious. After all, why wouldn't they? It's not illegal.