@David-TP My ISP is T-Mobile (Metronet). I did find and manually update the firmware and that seems to have resolved the issue. It's been consistently getting 2Gb up/down for the last five days.

I would still like to figure out what was causing the issue, just for sanity and if it ever arises again. 

A quick restart initiated through the app would not restore the speeds, but if I swapped it out with the ISP provided Eero, and then swapped it back, it would temporarily have full speed and then drop after a short while. I'm assuming a hard reboot/power cycle would have the same effect. For testing internet speeds, I only use the Deco app "Test Internet Speed" option. I do run speed tests from other connected devices, but am aware, especially if on wi-fi, that it won't reflect the true speed.

I am a 25+ year IT professional, so this stuff isn't new to me, but I am also not a network expert. This particular issue is baffling to me. I could see if the speed started slow, but not when it gets full speed and then suddenly drops. Seeing how the firmware resolved the issue, I can only specualte that it was a software issue and not defective hardware. It's bothersome that this could have been an issue since I've owned the devices until this most recent firmware was released. My next step, if the firmware hadn't worked, was to perform a factory reset of all three devices. That may have worked as well, even on the older firmware.