Unfortunately I've been hit with a similar issue many others have experienced. Here's a quick breakdown of my particular problem.
1. Purchased Gigabit service (Asyncrhonous 1000/35) and my own Netgear CM1000 modem in Feb 2020.
2. Reliable speeds of 900+/35-40 consistent tests in Feb, Mar, Apr of this year.
3. Since May 2020 download speeds crawled to 200-250mbps. Upload remains 35-40 mbps. Performance will sometimes completely stop 3-4+ times per week.
4. All components of my network are gigabit compatible. New RG6 coaxial, even a more recent new gigabit router, Cat6 cable etc.
5. There is no splitter between the wall plate and modem.
6. Speeds were also tested with a direct line. Reminder, speeds were fine prior to late Apr/May.
7. Spoke to more than several agents who promised a tech to come out, return calls, etc but to no avail.
8. Recently after 2 months of contact a technician came out and determined the following...
A filter was put into place by Comcast maintenance at an uknown date around late April or early May. I live in an apartment and the speeds were satisfactory prior to the filter. The technician stated that the filter is obstructing quality of internet speeds for myself and anyone else, especially those on gigabit service plans. He cannot remove the filter and suggested I simply downgrade my service.
This is unacceptable. Comcast finally credited me for the loss of performance but I would still like to receive the proper performance I do pay for. It makes no sense whatsoever to impair many residents of this area, myself included, especially without notice to the change and expect them to downgrade or "put up with it".
I've since requested another technician because there were some questions I simply forgot to ask.
1. Can the signal be amplified? If so, by how much?
2. Can I either install or replace a filter or splitter inside my residence myself?
The technician wanted to enter my premises but due to COVID-19 and having an immune compromised family member I cannot allow them to. I'm not averse to fixing the problem myself if that's an option. Even if I have to purchase a filter or other component and open up the wall plate to take care of it.
My question is what other suggestions, or questions should I ask of the technician?
I have literally that same issue. Same time frame and everything. However, I did have a tech come and test and he did not find a filter anywhere in my building, but he did test with some device he had and said he was getting 940mbps. I can't get higher than 450mpbs with any device directly connected to the modem (Mac, PC, Apple TV). Prior to mid March I had no issues getting 940mbps. I even bought a new modem to rule out my personal MB8600 modem being the issue. I got the approved Netgear modem and it did not change any speeds. I have all new top of the line equipment and wiring.
Still no solution or any sort of concern from Xfinity. I am in Minneapolis.