Customs in Japan is quick when you know which line you are supposed to be standing in, and slow when you do not.
Everyone declares
All passengers entering Japan must declare their belongings on a customs declaration form. There is no version of this where you skip the form.
Choose your channel deliberately
Use the green channel if you have no goods subject to duty, tax or restriction.
Use the red channel if you do. Duty and tax are then paid at the bank inside the customs inspection zone, rather than at the counter itself.
If your bags are not traveling with you
Where you have unaccompanied baggage, the customs declaration form is submitted in duplicate at the time of entry rather than once.
That second copy is what lets your delayed or shipped baggage clear later, so it is not a formality to skip.
One category with no allowance at all
Hunting guns, air guns and swords cannot be brought into Japan without a possession permit.
On allowances
Japan does publish duty-free and tax-free allowances covering alcohol, tobacco and perfume, and a declaration threshold for cash and precious metals.
We are not restating the figures here. They sit in a table on the official page, and a number that matters this much should be read from the source rather than from us. The link below goes straight to it.