I’ve been learning Japanese more consistently for a few months now and something I’ve noticed that slows down the learning process is you don’t just learn a direct translation of a word, you need to also learn:
Whether it’s written in Kanji or hiragana/katakana
How to say something in a more or less formal way for different situations
Cultural aspects (e.g. in Japan it’s considered rude to be too direct, you you usually need to find a more roundabout phrase to communicate your meaning!)
Japanese is one heck of a challenge, but a worthwhile one!
Interesting question! I think it depends on a lot of factors, like the grammar and pronunciation of someone’s native language, whether or not they have studied languages with different scripts before and of course, the sheer amount of time you have available to study!