Hmm, as far as I can tell (my Japanese isn't very good either
- especially not for technical stuff) it's something about downloading being ineffecient until your "unity" (cache) folder contains at least 2 GB, and once you have at least 2 GB in cache downloading what you're actually looking for should become easier.
(Ie. PD downloads lots of stuff, even things you aren't requesting it to download - having it download and cache lots of different things and having them encrypted in cache provides some form of plausible deniability I believe: It's not possible to tell from just what is in the cache what you yourself have wanted to download and what you're just acting as an anonymous carrier for. Of course IANAL and I don't know if that really makes things any better or worse in court if anyone decides to play those games.)