Probably they're overselling at a much worse ratio than I described, or there are other problems in their infrastructure causing poor performance (likely both).

Probably they're overselling at a much worse ratio than I described, or there are other problems in their infrastructure causing poor performance (likely both). It's not at all uncommon for ISPs to put even like 10,000 customers on a 10gbps line. 10 years ago that actually would have been fine, but bandwidth usage per customer has increased since then and naturally ISPs don't upgrade fast enough.