Please accept my apology for disagreeing with you. I have only recently become aware of the rules here and I now understand that disagreement is frowned upon and can lead to being banned.
The average vocabulary of the average person in their NATIVE TONGUE is just over 3500 words (it varies a lot from culture to culture).
The average vocabulary of a University Graduate in the Arts is 10-12000 words. In the sciences it varies greatly, chemistry, biology and medicine being the most demanding at >15,000.
Someone with a vocabulary of 20-25000 words would be highly educated or very well read, or an specialist in a field.
If you have a Japanese vocabulary of 8000 words, you already have a bigger vocabulary than 74.13% of the population (if you understand what that percentage represents, you already have a larger vocabulary than the majority of people).
>> There's just so much the brain can absorb in a short time.
Daniel Tammet learned Icelandic in one week. I would be interested to see what backs your claim that the brain can only absorb so much in a short time.
I think both you and the GP are projecting YOUR limitations onto everyone else.
The brain can and does absorb a massive amount of information each and every day. Training the conscious mind to direct that natural ability of the brain is something that is orthogonal to the brains capabilities. Being an Opera Singer (dedicated and focused) probably helps with that task.