First time through I didn't know what I was looking for so I missed a lot and had a couple false alarms. After a second attempt I caught everything though I was a touch slow.
The whole thing is heavily slanted against user success on the first attempt because unlike baggage screeners, most of us have had precisely zero training at this and the controls are unfamiliar.
With some training and familiarization of what you are looking for it gets a whole lot easier. How well you do after several hours of it? This test doesn't reveal. I'd run it like guard duty though, a couple hours on and then a good break preferably doing something that is not eye intensive (no tv, reading, video games, etc.)
I wouldn't want the job though, talk about boring.