I started a spaced repetition program for Mandarin, and I can see that it works well. Even though it's the fastest way, it still takes a discouraging amount of time and it is no fun at all.
I don't know if the process can ever be made any faster, but I am hopeful that AI agents will soon be able to at least make it a lot more enjoyable.
I learned French from 2021-2023 by taking weekly lessons. Before then I'd dabbled in all kinds of methods like Anki and tapes. Nothing works like having private or small class lessons with a teacher who can immediately correct you. Nothing. I went from barely able to speak or conjugate, to having conversations with my french colleagues, telling jokes, and reading novels, in French.
If you have the means, taking lessons is, at least for me, wildly more entertaining, fulfilling, and better than trying to go alone. I'm not affiliated at all, but using iTalki can really be a game-changer if you're trying to get conversational.
A language is used and using the language over and over in conversations is the best method for learning and getting better at one.
I don't know if the process can ever be made any faster, but I am hopeful that AI agents will soon be able to at least make it a lot more enjoyable.