Updating my post on 6/25 - I swapped the router connected to the internet and it fix my issue. When I swapped back, the problem reoccured. My conclusion is a faulty ethernet port on one of my routers.

is this an M5 only issue or does it also affect M9 - common firmware base? I started have issues with M9 significantly throttling bandwidth after upgrading to 1.2.8 Build 20190403 Rel. 43515 on June...

I have a similar issue with the M9 throughput that started about a week ago. I have ATT Uverse Fiber 1G down/up. Before, I would routinely measure up/down of 600-850 both ways through the Deco M9....