I didn't find anything about this, so I ran my own tests.

There is not much difference between loading times and dialogs before the battles, even if the USA is slightly faster on that for most characters, but the real deal is that Vegetto's perfect/little damaged victory speech is 2,20s faster on the USA version. And since you have to see that for 11 times, that makes the USA version 34s faster.

Here's the video proof:

very good to know, wasn't even aware that the US version is faster than JAP.
Thanks for clearing that out!

good to know