Reading all of it, I'm at the point to say it's very likely a hardware defect. That you bought from Google and got an authentic device doesn't guarantee this is not a rare case of a malfunction, a "Monday product" as we call it over here.
I'd be very frustrated if my top of the shelf device I paid good money for is underwhelming in performance. But since nobody else here (including me, I'm in fact very happy with my 8 pro) seems to have your issue, it's likely to be either your setup or your hardware. Your setup so far seems fine, no suspicious or unique apps you've told us about. And if it malfunctions right since your first setup (and you didn't mess with e.g. developer options, adb, system app permissions etc.) all I can come up with is a hardware defect.
If I were you, I'd flash back stock OS and send the device back to google, asking for a replacement device.
If you did mess with any critical things, I'd restore the default state (or do a factory reset since that might be easier) and try again.