I can't give any track-specific advice, because I haven't played MK7 in a long time, but being 15 seconds from the World Record, make sure you are collecting all 10 coins, using the correct kart/character combo, taking tight lines on all turns and corners, and maximizing use of glider (in MK7 you can swoop down and up to gain speed). I would advise watching videos of top players' gameplay and even reaching out on discord in the MKLeaderboards server (mkleaderboards.com has the link)