How do you test the speed? through Deco APP? wired connection from pc to Deco? or smart phone?
If the Deco app speed is also slow, connect PC to Deco and modem via cables to compare the speed.
If the wired speed from pc to Deco is slow, check whether there is any other clients connected or any other applications.
How about the negotiation speed?

Since you mentioned the speed get slow once you add the last unit, try to exchange the location of the RE devices.
Or try to set the last unit as the main router to see whether the issue still exits.

DesertSong wrote

Model :

Hardware Version :

Firmware Version :

ISP :

I bought a brand new Deco M5 3-pack. Works wonderfully! But I would like to extend the signal to my driveway by putting a 4th unit in the (attached) garage. While I was watching prices, I found a used 3-pack for sale - same exact thing I already have. The price was good and I bought it. It seems to have been well maintained, and the former owner states it was used as normal, just replacing to go to a different solution. But heck, now that I have 3 new units, might as well hook 'em all up, right?I have gotten all three onto my network with no trouble. But I was seeing some funky speed test results. Basically some really slow results in certain locations, although not always consistent. I disconnected one unit and then another and the funkiness continued. When I disconnected the last unit, my rock stable wonderful WiFi returned. What could the issue be? Does the unit have something wrong with it? Was its location too close (a room away) to another unit? Some other third thing?Ideas are appreciated. I have a 40/40 fiber optic connection. Normal speed tests show 32-39. The abberations I was seeing intermittently were like 8/2.

I have one network with no issues, you need see if each spot have signal to join at network, remember foi full speed at least 30db of SNR is mandatory to avoid speed issues.
use any wifi analise to see how your SNR at each point.

Let me see if I can answer your questions as best I can. I am relatively tech savvy, but this system (and mesh systems) are new to me.

When I had that "new" unit hooked up, when trying to run a speedtest through the app on my iPhone, I got an error. I don't remember exactly what it said, but something like it could not detect the Deco system - even though I still had internet connectivity. The numbers I reported (8/2) were from the Speetest (Ookla) app. Which is what the Deco app says it uses anyway. Although I suspect the Deco app inflates the numbers, because normally (without that extra unit hooked up) it tells me I am getting 38/38 no matter where I am - even on the corner of my property farthest from my house, whereas in that same spot, the Ookla app tells me I am getting 12/4.

I am not sure what the "negotiation" speed is. And I didn't have anything wired to test with. I haven't hooked the extra unit back up again to do more testing and the network ran fine all day. I will do though, maybe tomorrow and see what happens. It is interesting to note that during the time I had the extra unit hooked up, my wife was also having internet connection issues on her phone. As soon as I removed that unit, her problems cleared up too.

The unit was definitely placed in a location close enough to another unit (the main unit in fact) - only one piece of drywall between them - the main unit is in an unfinished utility room one room, and about 15 feet, away from the extra unit.

I did a little more testing and 2 of the extra units seem fine. I placed one in the garage, which was the purpose of buying them in the first place. After bringing it onto the network, I ran several speedtests, both using the Deco app and Ookla app. They tested out fine. Then I did the same in my master bath, the room the farthest away from any of the Deco units - it's not that big a house btw, I am definitely overkill here but that's not the point - brought it onto the network and ran several speedtests. Started getting the wonky results. Not quite as wonky as the first time, but speeds went down from pretty consistent 35/37 to sporadic 35/32 and 26/35 and 31/34. Nothing nearly as drastic as the other night, but slower than the norm and once in awhile the Deco speedtest would say "failed" like the other night. So I removed it and brought the last unit onto the network. It seemed fine too. My speeds seemed to drop negligibly, like from 35.2/37.4 to 35.0/37.1 - very small I know, but sort of consistent; I attributed this to the fact that more units were using up more bandwidth with the way they do backhaul using the same WiFi frequencies as other devices. But in any case, no more sporadic behavior.

So I put the remaining unit aside, and noticed it's after midnight and I will do more testing tomorrow. But the evidence would seem to say one unit has something wrong with it; I am just not sure what that something has the possibility of being.