> you will need to spend 10x effort than an ordinary Chinese kid

I really don't think this is the case. An ordinary Chinese kid is learning Chinese 24x7 -- for decades. If you start learning a new language when you are an adult, the expectation seems to be that you'll be both fluent and proficient in a few years. There is a myth that children are able to do this.

It's just not true. Talk to a 5 year old. They have mediocre fluency, and very little proficiency. The average English speaking 5 year old only has a vocabulary of 5000 word families. Contrast that to the average high school student with over 15,000 and the average university graduate of over 20,000. Even by 10 years old, try plonking them down in front of the news and ask them to summarise what was said. Most 10 year olds can't do it, because they lack both vocabulary and grammar. Look at English text books for junior high school students. Note how basic the English is. And still many students struggle to comprehend (though by the time a child is 10, they have pretty amazing fluency because that's what they practice all day, every day).

I guarantee that you can vastly outperform a child in learning a second language if you spend comparable amounts of time and effort. I say this having taught English as a second language.

Now you may ask why, after years of study and frustration in classes that most students can produce no language. The main reason: if you don't review every day, you will hit a ceiling pretty fast. Students usually have class 1-3 times a week for 1-2 hours per class. This is literally useless. 5 minutes a day, every day, will give you dramatically better results.

The next biggest reason: teaching fluency is unpopular because historically many teachers never achieved any real fluency (and they inflict the poor teaching they received on the next generation). Also, fluency is virtually impossible to test objectively and takes a huge amount of teacher time to even evaluate. I worked it out one day. I was expected to work in classes where my opportunity for 2 way conversation in the class worked out to less than 30 seconds per student per hour. Pretty much guaranteed failure.

Next biggest reason: Students are told that 2000 words of vocabulary will make them fluent, and so limit themselves to this level (that is surpassed by 3 year old native speakers -- talk to a 3 year old and judge for yourself). Students also forget the frustration of being a child and not understanding what's going on. Children frequently burst out in tears and tantrum all over the place because they are so frustrated. We have this idea that the acquisition of our first language was effortless, and so wonder why we need to spend so much effort in the next language. Next time you talk to a child, question why they don't understand what you are saying. Very often the surprising answer is because they don't speak English very well!

And finally (finally for this already long email, not because I ran out of reasons): Adult speakers are unwilling to act like children. They are unwilling to put themselves in situations where they do not understand what's going on and where they have no control over their surroundings. They are unwilling to be silly, and to make mistakes and to interact with language resources that are aimed at their level (like cartoons). Again, watch a 4 year old. Copy them. (The same Spongebob episode 500 times in a row until you are repeating the lines to yourself as it is playing). It's not because it is harder as an adult. Just the opposite. As adults we are so used to having thing be so easy that we are reluctant to put in the effort of a 4 year old.