I heard somewhere that things learned quickly are forgotten quickly. Things learned slowly are retained. I suppose this theory does not really apply to most gifted children? 8th grade fill-in-the-gap summer study is reasonable?
That theory is more trite than real for anyone. At best it describes disconnected cramming.
We learn best at our level challenge, when ideas come in and connect with things and we work a bit to make sense of it. When the information is engaging and interesting, we put more focus and more of our mind on it at the time; the boost of the thrill of succeeding also makes the recall better. If you're floating in a bored zone, your brain engages less and encodes more shallowly; active attention is the currency of memory.