For local connections I think you're spot on. A lot of ISPs had massively over provisioned backhaul. With a larger than average number of users connected simult

For local connections I think you're spot on. A lot of ISPs had massively over provisioned backhaul. With a larger than average number of users connected simultaneously and heavily using their connections there was not enough backhaul to handle the load. This could be even more pronounced on cable since a single segment could have hundreds of subscribers sharing that segment's bandwidth.