I took a similar test but in person at an office a few years ago. Exact same layout, of "click when x appears, do nothing if not". It was only 25 minutes I believe but felt like an eternity. I had a 100% success rate and above average reaction speed, but I still "failed" because of my observed behavior as I was taking the test.

The proctor noted my restless legs, frequent posture shifts, and "tendancy to tap the table when not responding to the stimuli" (this was a strategy I had come up with to create something to do in response to "doing nothing", as it made a sort of rhythm that gave me something to do). I then realized that the test meant very little, but the proctor was watching and diagnosing me durng the test.