Victor’s brilliant comment deserves a call-out:
The same can be said about class work and research. Even on the same topic or in the same academic area, someone can be good at one and struggle with the other.
However, in writing about intelligence, you have to be a bit careful, from my experience as an educator. What you grasp quickly (which means what you grasp now) may be unrelated to intelligence and rather related to what you have mastered, holes in your knowledge, your experience, and your past. Those factors also affect what you do not grasp quickly or with what you initially struggle.
Secondly, what you say makes sense only if you are talking about where people are at this moment (what I know, what I don’t know, what I struggle with, what I’ve mastered). If you use the word “intelligence,” rather to refer to people’s potential, then that changes everything. This is partly because a theoretical physicist can learn to do experimental physics regardless of how she is today. An electromagnet physicist can learn to be really good at electronic circuits even if he is bad at them today.
Thirdly, I tend to use “intelligence” in the latter way. However, if you use it in the former way (I believe this is how you are using it), you have to remember that that “intelligence” or “smartness” or whatever it is called is not static but completely dynamic and malleable. In fact it is so malleable, even with the most struggling of students, that I tend to think a better definition of “intelligence” is the potential-based one. It’s so changeable that what people usually mean by “intelligence” is just current understanding and ability.
If I haven’t said this before, I have much to learn. And especially from all of you.