RE: If you have a dual band simultaneous WiFi router/access point, try connecting the Google Home Mini to the opposite band that the computer is connected to.
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You may be onto something there.
I have both 2.4 and 5. All gadgets are set to default to 5G - except the printer is set to 2.4 (which is rarely used).
I will play around and report back.
UPDATE : BINGOUsing the Google Home App, I "unlinked" the MINI from 5G and added it to the 2.4G so that only the desktop, laptop and iPad use 5G by default, when on.
The printer and MINI are linked to the 2.4G network. (I can temporarily switch to 2.4 whenever I want to print from desktop or laptop wirelessly through Wi-Fi.)
Backing up with Time Machine (on 5G) has returned to its usual speed.
P.S. I will chime in again if these changes don't "stick."