What is the role of this Zener diode in a LED string? (Old traffic light restoration)

Yes this is a real thing and it seems to effect the rate you perceive the passing of time. From a BBC program https://www.bbc.co.uk/programmes/articles/2xFf4fxJmxzyV9XWjf1J886/does-time-move-faster-as-we-get-older

Another oddity of the brain I have not seen documented anywhere is that if I blink rapidly as I can, things seem to be in slow motion. I discovered this watching cricket as the bowler makes his run up.