The first thing you need to know is that PAL (Europe mostly) is 50fps (50hz) while Japanese and NA (NTSC) are 60fps (60Hz) and so, the game will obviously be slower for the PAL release, even if the cars has better top speed. Because of that, don't use this version of the game if you want to be the fastest.

The main difference between the NA and Japanese releases is that when you press start during a dialogue on GP Mode, it will be skipped entirely for NA when on the Japanese release, it skips only for the next part of the dialogue (same thing as pressing X). Because of that, since skipping the dialogue is alot quicker on the NA release while the game is the same, the last one became an obvious choice for Any% GP Mode, while you can pick JP or NA for Time Attack.