Types of Giftedness
But a lot of people don't understand that gifted kids don't really learn faster, instead, they learn more efficiently. Meaning, basically, their mental processes aren't the same thing sped up. They are actually learning in a qualitatively different way, a way that allows them to process more information in the same time period.
In my impression, there seem to be two common types of talents resulting in a kid being considered gifted: memorization and comprehension.
Imagine a kid, considered gifted, who has normal comprehension and better memorization. If you got this kid and a typical kid and gave them both some information that was completely new to them, they'd both understand it about equally well. But test them two days later, and the gifted kid might remember it while the typical kid's completely forgotten.
Furthermore, this memorization ability helps them learn new information, too. Imagine teaching those two kids a second lesson that builds on the first. The typical kid will probably need to be reminded of what xe learned in the first lesson, basically retaught the first lesson (though more quickly than the first time). The gifted kid will still remember everything in the first lesson and won't need the reminders, meaning you could spend that whole time on the second lesson.
Secondly, regarding comprehension, imagine a kid with improved comprehension but normal memory. When this kid and a typical kid are taught something new, they'll both remember what they learned about equally well. But in the first lesson, this kid will need less spelled out explicitly, and will be able to extrapolate more on xyr own. Therefore, it would take less time to explain the same concept to this kid.
And at the second lesson, both kids will need a recap of the first lesson. But when you're reexplaining something the kid has learned but forgotten, you can do it a lot more quickly, and chances are that the gifted kid will have figured out part of the second lesson on xyr own during the first lesson. This means that part of the second question will be recapping stuff the kid already knows, and could be done very quickly. The rest would still be quicker than for the typical kid, for the reasons described above.
Incidentally, the first type is more common among high-average and mildly gifted kids, while the second type is more common among highly gifted kids. In addition, some kids have above-average memorization and comprehension, and both of those skills feed into each other (you remember better if you understand the material, and you understand it better if you remember related material). Lastly, not all gifted kids can be described this way. In particular, some gifted kids seem to be just more motivated to learn, and this alone could possibly explain their giftedness (or high motivation could combine with improved memory and/or comprehension).
Anyway, the point is that if you understand how a child is gifted, you can teach them better. A kid with an excellent memory but poor comprehension needs a whole lot less review but only slightly less explanation, while a kid with excellent comprehension and a poor memory needs only slightly less review but a lot less explanation. In addition, you can use memory to build on comprehension and vice versa, by explaining how the material you're teaching relates to things the kid already knows. This will help the memorizer understand, because they remember their understanding of the previous material, and it'll help the comprehender remember, because they can fit this information into a web of linked concepts.