@Jdoc 

Swap out the main Deco with one of the slave Deco units, then test the speeds again. 

Power down the slave units, and focus on the main unit to test the speeds. If speeds are still slow reboot the units, test, if still slow default it to see if the speeds come back.

@Tony 

thanks for the suggestions.  I was finally able to get it to work, but it required a complete reset of everything.  It's the first day after getting it to work, so I don't know if it'll last but for now I have 110mps near each of my three deco p9 devices and at least 60 in between them.  I get 110mps via ethernet cord direct from the modem for reference.

I tried removing devices from the network, switching the main deco connected to the modem, removing and adding devices, and many other things.  Nothing worked.

What worked was deleting the entire deco wifi network in the app and starting set-up from scratch.  Then, before starting the set-up of a new network I set each device one by one to factor default.  You have to plug them in, wait for the solid yellow led to pass (that's the booting up color), then when it's flashing blue or red or solid white push in the reset button on the bottom with a paperclip for about a second or two.  Wait for the device to flash yellow, go solid yellow, then flash blue.  Let it flash for a bit then unplug it for two minutes.  Plug it back in, wait for solid yellow to turn to flashing blue, wait another couple minutes, then hit the reset button again and wait again for flashing blue.  Then unplug it again and go to the app and follow instructions for setting up the network from scratch.  This all takes a significant amount of time (particularly resetting the devices to factory), but it does work (at least for now).

Hope that helps someone else.