Download speed tapers off during speed tests

Has anyone else seen this? Our internet has been horrible for the last week. We average about 20mbps download speeds and we should be getting around 200. When we run a speed test, you can see the meter jump to around 20 or 25, and then as the test goes, it just slowly tapers itself down. Normally when I run speed tests the needle moves and jumps all around, I've never seen this behavior before. It's like it hits a ceiling and then someone turns it down, like you might slowly turn down the volume on in a stereo. It's a steady, measured, decrease in speed while the test runs. We have children doing school from home during the day and I'm supposed to be working at night. We've had such severe Interruption of service we aren't able to do these things. Xfinity's solution is to come out this weekend which will be four days after we placed our initial call, and doesn't do anything to help us do work and school & have everyone online at once. So they gave us a $21 credit. Very disappointed. We've done the usual basic troubleshooting, modem router resets, connecting with ethernet directly to the modem and testing on that machine and not over Wi-Fi. Then they sent us a message saying they were doing maintenance and actually found a problem on their end and did we still want the tech call? Yes, we do. We also lost cable service the day we placed the service call, when things were really bad.