Fats do take longer to convert to energy. For most people, the time taken for food to be converted into stored glycogen (the fuel for muscles) is not a factor. For an athlete, glycogen is stored in the liver and in the muscles. It takes about 20 miles of running for these stores to be depleted (hence the 'wall' in marathon running).This is unlikely to happen for a dancer or a sprinter. But for a top tennis player over 5 sets, it could. Hence the banana, which also provides important minerals to help keep the muscles working.
I wouldn't recommend white bread under any circumstances (except i believe it can be use to erase a soft pencil mark). Glucose drinks are best if you want really fast energy boost. Quickly absorbed and converted to glycogen fast. (chocolate is good too

But to be honest, as a dancer and a mad exerciser myself, my view is that it's best to eat a good balanced natural food diet as much as possible.
Eating fast energy release foods (so-called high G-I foods) can stimulate insulin production to convert the sugars to stored fat.
