Assume for the moment that there was some kind of intermittent connection someplace, it wouldn't account for the speed slowing down. It would go on and off. You would lose connectivity just like you pulled the plug out of the jack. No such thing as half way. Your computers would indicate no internet connection or no network connection.No, we are definitely talking about the network wiring here. I was called because I had installed CAT6 to several rooms. After some troubleshooting, we narrowed the problem down to the modem to router jumper which was existing except that put a new connector on one end. A cable tester showed a "pass" condition. All the new cables I installed passed also, but I double-checked the punchdowns just in case.
It's amazing how an electrician always suspects a wiring problem when there is a problem with computers and internet.