>>45874891X and Y was already pretty bad on anything that wasn't a 1 on 1 battle, and even then it had a tendency to chug with certain attacks.
Sun and Moon added a bunch of extra shit like seeing your trainers in battle and these trainers models being way more detailed in general and now the game is shitting the bed at pretty much every turn even in the most basic of battles.
This is why "future proofing" the Pokémon models probably wasn't that great of an idea, but I guess we're past that now and on much better hardware where the games can look and run as good as they need to, right?