That was a most odd experience on the Men's Shed 2020 iMac using Vuescan. The app was failing to reliably detect individual objects. If I put 4 photos down on the scanner bed, in most cases it would only detect two of them, sometimes only one.
So I finished that first batch of 25 pics one-by-one. Even so, I had to force-quit the app several times as it got hung up on a scanning loop, outputting pics previously scanned while ignoring those detected with the preview scan.
The force-quit keystroke combination of Command-Option-Escape revealed two instances of the Finder . Quitting either one forced both instances to quit before reappearing.
Restart time. Checked the force-quit options, Finder is only there once.
Tried four pics on Vuescan again. Worked perfectly before announcing an update was available.
The developers (Hamrick Software) seem to issue new versions at the rate of one per month on average so I installed the new version. No discernible difference, no problems with individual photo detection.
I timed the process of scanning four photos at 600 dpi. From loading the scanner to removing the pics after being scanned took two minutes, the actual scan was 20 seconds per photo, which is at least double the rate of the scanner’s “proper” software.