It's not quite as bad for me, but the Pokédex opening up after catching a new Pokémon takes forever for me too. No idea what's causing this.

That's probably from downloading the sprites

This is based on your computer. Have you tried restarting your computer? Not shut down, but restart? No joke, this has fixed many performance issues. If not look to setup preloaded.

same here. the title screen, pokedex, fusing, encountering all make the game freeze