My question is...how do you eat it within 30 seconds? I get that this type of etiquette exists in many different cultures but while I have never eaten sushi, I don't exactly get how that one is even possible?
The context of this sign being in a sushi restaurant would be the key here. In higher-end, "omakase" sushi restaurants, you'll be served a set of sushi piece by piece as the chef makes it in front of you. Typically you'll want to eat it as soon as it is placed on your plate.