From my experience, it's a delusion. The focus is there for some while, but the actual abilities decline very fast without you even realizing it because of this supposed clearness. It might be even that the clearness comes exactly because of your mind limiting itself to a shorter attention and horizon, removing all the complex and complicated things. It's basically being in the zone, but the zone itself is so limited that you might not do good work if you need to have a "big zone". Though, it depends on your type of work if this can be beneficial. But as a knowledge worker, I consider it harmful for my work.
> When I was a kid, I recall the teachers always telling us to have a good meal before a test. Even then I felt that was silly, as a big lunch always gave me brain fog.
Depend on the type of food, size of meal and how long before the test you were eating.