I am having the same issue. I've had techs come to the house, I've driven an hour to the Xfinity store and swapped the modem. Nothing changes. For the past three days, the wifi has been SO SLOW! Usually, my phone will register 200 mgbs on wifi, but now I get 5 mgbs standing in front of the modem. The computer that I have in the same room will measure 12 mgbs on wifi and a whopping 20 through ethernet. I have used the "troubleshooting" app, been on hold for 1+ hours, and nothing ever changes. It is supremely frustrating, and today I have lost a day of work due to speeds so slow Zoom won't work.