Reading this on a Nexus 5X running Oreo 8.1.0. Formerly happy owner. Now, after accidental pocket-tap on "Update All", seeing glacial performance in core apps e.g. Gmail, Maps. No "revert" option in GUI of course. Maybe possible with ADB ...

Where you write "stopped upgrading", do you mean hardware only -- allowing Google system plus apps to push all software updates?

How do you assure yourself that your Nexus 5 has gotten adequate security fixes?