My wifi speeds are wildly inconsisted from one second to the next, even when standing or sitting in the same spot.  My speed will be 200 mbps one second  and then plumment to 50 or 30 or 1 mbps 30 seconds later.  My hardwired speeds have fluctuated somewhat but generally they are over 200, so I am tentatively ruling out a defect in the line to the modem.  I'm using an iphone 11 and ipad pro to test the wifi speeds.  I have been speed testing like a crazy person and cannot correlate the fluctuation to anything in the environment. 

I have a Deco M9 plus purchased from Costco last month.  One story home with two people currently working from home but neither one is using much bandwidth except for the occasional zoom or Microsoft teams.  The main unit is in  "home office 1"  hardwired to the modem and a gigabit switch.  Connected to the switch is the first computer, the second computer in  "home office 2" and the second deco in the living room.  I have tested the speeds of the living room deco with a line directly into my computer and it is putting out 200+ hardwired.   Because the hardwired machines are relatively stable I am tetnatiely ruling out defects in the cat 5e cables from the modem to the main deco or from there to the switch and beyond to the hardwired computers or second deco.  

Home office 1 and 2 are across the hall from each other -  about 36 feet from the main deco to the far side of the room  across the hall with no obstacles as long as the doors are open.   Most of my speed tests occur sitting in my chair across the hall from the main deco; however, I have expereinced the same issue in the hallway between the rooms.  I can stand over the main deco and get speeds of 200+.  I can walk across the hall and test the signal over and over again and get speeds of 200+ all the way to my chair and even sitting in my chair.  1 minute later the speed in my chair might be 50 or 80 or 1.   I can walk back to the main deco and test along the way and continue to get low speeds until I am standing over the main deco.  The speed will revert to 200+.  Or the speed will revert to 200 somewhere along the journey back towards the main deco. 

The second hard-wired deco can produce wifi speeds of 180 -200+ when standing over it and as I walk to each corner of the living room.   The speed slows as I walk into the kitchen or into the backyard but can still break 100 mbps.  However, this deco also randomly does the exact same thing as the main deco - the speeds will plummet for no apparent reason fro one second to the next. 

I've tried this with fast roaming off and on.  I have tried it with my phone priorities in QoS and with no items prioritized in Qos.   I tested it with the two hardwired units on and the third unit turned off and with the third unit turned on.  I can't find a cause or a cure.   Anybody out there have any advice for me?