[deleted] Certain parts are very manual, such as putting the device online so it can enable OEM unlocking; enabling OEM unlocking; rebooting to bootloader; confirming bootloader unlock; confirming bootloader lock. Those seem irreducible in terms of time.
The flashing itself takes a while. Maybe that could be sped up or parallelized, but connecting multiple devices to one computer via a USB hub might increase the rate of flashing failures.
Maybe lots of flashing stations? This isn't like bulk-copying USB thumb drives.
Google likely has ways to do it quickly, but they have privileged access to the platform.