On what computer? Actual modern macOS updates takes some initial time indeed, but in the background! The reboot phase is not very long.

As far as I can tell, the updates that require a reboot don't do anything in the background now (prior to the reboot), besides downloading.

They do much. They spend usually around 20 minutes doing stuff before rebooting.

2018 MBP.

Ha yeah, IIRC macOS updates on Intel are slower (and not only because of the processor and/or disk, but because of the whole architecture).