I doubt that one granny smith apple will affect your energy, although it might make you feel better, which might in turn improve performance. I don't think that it will be significant in terms of the bad effects associated with overconsumption of sugar. There are a number of implied sub-questions in your post so the answer is complicated:
Will eating an apple help athletic performance by providing calories?
Probably not. During intense exercise, such a small amount of calories would be expended very quickly, so it's not likely to contribute much to fueling movement. In fact, eating during exercise can be counterproductive, as digestion diverts blood supply to the stomach instead of to the muscles. Generally commercial "athletic fuel" formulations are designed for quick digestion, made of liquid or gel with simple molecules like glucose and amino acids, and (I think) the more solid or fibrous the meal, the harder it is to digest. I believe these kind of fuels are only shown to be effective in long endurance events lasting multiple hours (e.g. longer than a half-marathon), where the body's glycogen is significantly reduced.
Will eating an apple help athletic performance by sating hunger? I doubt it. It's true that your feeling of hunger may be relieved by eating something, but in my experience that can be ignored while exercising and has no impact on athletic performance. I would much rather feel hungry while training than feel like food is sitting in my stomach.
Is there any way eating during exercise can improve athletic performance? Possibly. In spite of the above, there may be a psychological or morale benefit to eating something like this. If an athlete is in the habit of never allowing a feeling of hunger to exist without immediately eating, or if the athlete is convinced that lack of food is degrading performance (even if it's not true physiologically), it could be possible that eating even a few bites would enhance the ability to perform.
Is an apple more natural than commercial products? Yes! "Natural" is a vague term, but my impression from looking at the cliff bar label is that it's made out of a long list of seeds, sugar and chemicals just like most any candy bar. The apple is definitely more natural than anything in a package (of course that doesn't mean it's better for you - tobacco is natural too).
Is there anything I can consume that is natural and fills the stomach, which provides a feeling of energy without making me slow down? Yes. Water, tea, coffee are obvious ones. Pickle juice is one that is not so well-known but has lab research behind it (Phys Ed: Can Pickle Juice Stop Muscle Cramps? - NYTimes.com]1).