Glucose is a monosaccharide which means that it can be absorbed without any chemical processes and before a competition it can be metabolized faster. Bread contains starch which is a polysaccharide. That means it has to undergo chemical processing in order to be absorbed.
Glucose⇒intestinal absorption
starch+amylase⇒maltose+maltase⇒glucose⇒intestinal absorption
So, to summarize glucose takes it's effect faster.
