Mario Kart games have always been known for the borderline exploits you can use to squeeze out the fastest lap times possible, with firehopping being the Mario Kart 8 version. That technique, where you rapidly hop while switching directions in a boost to gain a speed advantage, has by design or accident, been negated in the new 200cc mode in the game.
NeoGAF made the discovery, which has yet to be confirmed by Nintendo or video analysis but anecdotally something has changed in 200cc that reduces the effect of firehopping. Some will lament their advantage disappearing, others will appreciate the game returning to a focus on lap times and item management rather than pulling off tricks. Note that this only affects the 200cc mode, leading me to believe it is an unintended side effect of the speedy new mode rather than direct action by Nintendo to remove the exploit.