Orbi recently became very slow.

What is the Mfr and model# of the ISP modem the NG router is connected too?

Arris SB6183 Surfboard

After doing a FW update, was the ISP modem and Orbi system powered OFF for 1 minute, then power on the modem and let it sync, then power on the Orbi router then satellite and try a speed test via wired PC?

The cable modem was, the Orbi was not.  I only powered the Orbi off when testing with the 2.1.4.10 firmware.  I will try this once I finish this reponse.

Is QoS enabled on the Orbi router?

Both "Enable WMM (Wi-Fi multimedia) settings" and "Enable Upstream QoS (Optimized for Gaming)" are enabled.

What is the distance between the router and satellite(s)? 30 feet is recommended in between them to begin with depending upon building materials.

All this testing was done on an ethernet wire, not on wireless.  I intentionally avoided wireless tests to rule out Wi-Fi related sources like dead spots, materials, etc.  When I do  the test on my Galaxy S9 sitting 3 feet from the Router (no obstructions of any kind), I get the same poor results.

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.

As already noted this was all done on a wire (Cat5E), not wirelessly.  However to cover all the bases, on 2.4GHz Im using Auto and on 5GHz Im using channel 48 (unused).    I will try the changes you suggest later if you think it relevant even though I'm using ethernet.

Any Wifi Neighbors near by? If so, how many? 

There are 2 with Wi-Fi on.  Both of them register signals of ~-85dB vs my Orbi at ~-40dB on 2.4GHz.  No neighbors are on 5GHz.

Try disabling the following and see:
MIMO, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Save settings and reboot the router and satellite(s).

Thanks for the suggestions.  I will try them after I redo the power cycle dance for a full minute on both the cable modem and the Orbi (all with Wi-Fi disabled entirely).

After doing a FW update, was the ISP modem and Orbi system powered OFF for 1 minute, then power on the modem and let it sync, then power on the Orbi router then satellite and try a speed test via wired PC?

I powered off both the modem and the Orbi for 5 minutes while I went and got a drink.  I then followed your steps and got only a marginal speed improvement.  I now measure ~30M instead of ~20M when using ethernet.  My ping times jumped from 5-7ms to 28ms.

I also did your other changes to MIMO, etc.  I only see a checkbox for 20/40Mhz Coexistence under 2.4GHz which was already checked.  I assume thats what you meant by "40Mhz only".  IPV6 was already disabled.  I also disabled SSID broadcasting on both 2.4 and 5GHz since I could not find the setting to totally disable Wi-Fi.

One new finding to report is that the Upload part of the tests fail now:

A socket error occurred during the Download test. A firewall could be blocking the connection or the server might be having some issues. Please try again later. 

I retried 4 times and always get that same error.  My earlier tests did not have that problem.