Come imparare le lingue in modo rapido e efficiente?

The most effective method is SYSTEMATIC and PERSISTENCE.
As you wrote, one goes faster, the other slower.
I recommend the book "Breaking in to the brain". The author conducted a series of experiments on himself to test various methods of learning. All this in order to select effective learning techniques. With their help, he wanted to master a randomly selected foreign language within 6 months so that he could communicate in it without any problems and pass the exam organized by the university. Fate found a completely unknown Swedish language. THE EXPERIMENT HAS SUCCESSFUL (Total Test Score 87.5%), and its details are detailed in this book.

Hello, there is no universal quick method to learn foreign languages. It is important to define the purpose for which learning a foreign language is desirable. Stimulating motivation and maintaining it at a high level is one of several elements necessary for acquiring various skills, including learning foreign languages. It is worth determining what is the communication need ?: private trips, professional work, etc. The methods I recommend: contact with a native speaker, because it is an effective method, forcing the learner to communicate in a foreign language, although it is not a method for shy people or with low confidence. In the initial period, it is worth choosing 1-1 lessons with a Polish or native teacher.

Immersing yourself in the language! The best way is to learn basic words and phrases with an app (e.g. Duolingo) and then ...

Consume all content in the language you are learning. Listen to songs in this language. Watch films with a teacher, or better yet, those originally recorded in that language. Start reading the simplest books (often whole series are published for beginners of a given language. If this is not available in your chosen language, I recommend books for children).

The beginnings will be difficult (when I started, after reading one word of the book I had to check three different words in the dictionary), but this is the fastest way to act, because thanks to this you learn the necessary minimum, and you will understand the rest from the context :)

What language?
If any of the "basic" European languages, or Russian or Ukrainian, see the Institute of Linguistics.
instytutlingwistyki.pl - (you have to manually copy, because as I give "http" at the beginning, the link does not work)
Great learning methods taught by native speakers.
For example: a method of learning vocabulary by typing an unfamiliar word on Google and associating the word with an image (just like in your mother tongue).
If I were you, I would also check the conviction entitled "Everyone wants to learn languages these days." I had them too, and when I came to the point that I didn't need it at the moment (i.e. in my development something else is a "bottleneck"), I rejected this belief and stopped frustrating that I only know English for B2 + and hey ;-)

I believe that the fastest method is simply to function in a place where the language is used every day. It's not just dry knowledge, but the opportunity to get used to the language, the cultural and linguistic environment has a very big impact. I can see it in my example. I have been learning English since the age of 5. At the age of 22, I unexpectedly landed on an international university project dedicated to saving endangered languages. The idea was that linguists from around the world, representatives of national and ethnic minorities, members of associations for saving languages and culture work together during lectures, workshops and fieldwork. I was one of the youngest people there and didn't know what to expect. Initially, English was very difficult for me as a language of instruction (linguistics is a niche field), as well as working in groups (also in English). However, after 2 weeks of the project, I was able to take good notes, create a team project and talk about it in front of the audience. It seems to me that it's like this - you can't swim and you're thrown into the water, then somewhere in you the survival instinct kicks in. This is probably the easiest way to learn a language in a practical and useful form. In everyday situations, no one will ask you if you know all the rules that the teacher required, but if you can cope and function. Or in the case of work - can you work.

I've always been skeptical about distance learning, such as courses, etc. And yet ... It turned out that despite many years of failure, I found a school, or rather I bought an application that can't help but teach. during walking, rollerblading or training. What course am I talking about? The "Active English" course helped me.

The most important thing is consistency and regularity in language learning. Dedication to daily study and practice is the key to success. Additionally, it is worth immersing oneself in the language by watching movies, listening to music in that language, and speaking with native speakers. Practicing speaking and writing is also very important. Using a variety of materials and methods, such as online courses, textbooks, mobile apps, or lessons with a teacher, can also accelerate the learning process. The key is not only consistency, but also commitment and motivation to learn.