I assume the people at the top of the Nitro leaderboard aren't playing online matches to get those points. There's just no way. It has to be them playing Limit Battle Battle mode or something similar over and over and over again. What they are doing must be so mind-numbingly boring. I'll really laugh if some random(s) pop up at the top of the leaderboard due to cheating(save wizard). This happens to every offline multiplayer save data game with leaderboards on the ps4.
With the 50% Nitro gain bonus, online races only give 15 and all the lobby/loading/waiting down time makes it a far from efficient way to gain Nitro. Now there's non-online play in which any local battle/race/trial etc.... gives 7 Nitro. Using a second controller to play 2 player games by yourself where the 2nd player is idle has to be what they are doing. This way I was able to get 7 nitro every 30 seconds which is around 840ish an hour.
I wonder what the optimal method for farming Nitro is. Which game mode, track and settings?
