Why the Japanese version of the game is vastly used? Have major differences/minor loading times?

Has the least amount of text strings compared to any other version.

Q: Why is the game in Asian?
A: Full-game speedruns of Vice City are typically done on the Japanese version because it is one of the so-called "Haitian-friendly" versions (look it up if you don't know what that means). This means the mission script has been slightly modified from the original game, which in turn is relevant for different instapasses. The only other Haitian-friendly PC versions are the German Green Pepper version (no rampages and two cut missions, severely altering the route in a bad way and its 100% isn't considered 100% for leaderboard purposes), the Australian version (game crashes on Dildo Dodo and upon entering the Ammu-Nation menu), and the Polish version (technically unofficial and same issues as Australian version). You can technically use the Australian version for No SSU since Ammu-Nation isn't used in the route, but there's no real advantage to being able to switch languages nowadays anyway.