Ron -- I figured it out!!!!! I went back to the BIOS settings again, went through all the choices slowly,  and found a choice under "POST Behavior" that was not listed in that Dell 27-7700 Service Manual. It's "Keyboard Error" and is described as "Keyboard Error Detection."   I unchecked it, rebooted and now it boots up as quickly as without the dongle and with no error messages.   I have that extra USB port back     So this would be the solution in case you get this question from someone else. Thanks again for your interest.   Stay safe.   -Alan
I have a new Inspiron AIO and EVERY time on startup a "Keyboard not found" error message appears. This makes sense because I'm using a Bluetooth keyboard,  so at bootup the computer can't see it.  I understand that.    Once the computer boots up everything is fine. I've used many Dells with Bluetooth keyboards and have never seen this message before on other machines.   Is there a way to turn this message off?  I don't mind error messages when something is really wrong.  This is not a big deal, it's just irritating.   The computer is a Dell 7700 AIO.  Windows 10 Home.    
Inspiron 7700 AIO, keyboard not found error on startup