I will accept the outcome of the decision, so there's no need to be rude, hopefully. I just find this question to be generally useful to Japanese learners, as I've spoken with countless people who have the same question. I think it would be a valuable question to have answered.
The question, while opinion-based, is not unique in its inability to receive a unanimous comment section.
A list of examples on Japanese SE:
How one can learn something can have innumerable responses, as it did. There are countless ways by which to go about memorization, such as the Feynman technique, mnemonics, etc. Asking how one can learn and recall something is not only fundamentally impossible to answer due to one's unfamiliarity with the individual, but it is impossible to answer because there is no absolute consensus on the subject of memorization. Such a question is better left to neuroscientists. Yet it received dozens of opinionated answers about how to learn something, which is the essence of my question as well.
This question won't cut it because there are an extraordinary amount of schools in Japan. It's possible that some schools begin much earlier; some schools might have gifted or mentally disabled children; some schools might be richer or poorer, etc. One cannot say for sure at what age children learn kana/kanji, nor how they do it. Some schools and teachers, I'm sure, have different styles that make it incomparably different than others. So, the results of such a question are by its very nature opinion-based.
I won't go into length on this question, but see the above criticisms: too many schools, teachers, types of students, etc., to provide a consensus.
Which non-native speakers? Also, which kinds of people? Savants? They'll be unlikely to be troubled by it. What teaching style are we referring to, or memorization techniques? What if a book specifically targets gairaigo? One would be expected to pick up it rather quickly. How do we define "more difficult." The list of factors continues ad nauseam.
I do not think that any of these questions should be put on hold or closed, as I think they're useful questions that provide Japanese learners with a variety of options to help them learn. But my question, in essence, is not much different than the examples, and in some cases, it is far less ambiguous. In the same way that one can do their best to reach an informed opinion on memorization techniques, Japanese enthusiasts ought to be able to develop an informed, respectable opinion on how to best learn kanji and vocabulary.