Open Your Heart, Lava Reef Zone Act 1, Hydrocity Act 2, Speed Highway, Egg Reverie Zone, Studiopolis Zone Act 2, Chemical Plant Zone

I think it was to extend the player’s fun and the length of the game

Don't forget that in Sonic 2 and Sonic 3&K, Eggman was able to outrun Sonic

Sure, there's no difference at all if you use a 24Hz monitor