Why is my ‘net Speed in the Crapper?

So I recently called my ISP to complain about some service issues and billing disagreements. Since then, my internet speed has been noticeably worse. I used to get 70-80 Mbps throughput on my line, which I believe is supposed to be rated at 100 Mbps. My latency is typically in the 20-80 ms range. I can’t complain about those numbers. However, since my speed and latency has been so clearly worse than normal, I did another speed test tonight – the first I’ve done in months. Here’s what I get:

With later testing, the speed improved to single-digit Mbps, but latency still turns up in the mid-hundreds of milliseconds across repeated tests. And no, there are no downloads going on right now to slow things. I checked.