Lag when using 2nd gen mon

When I'm in a gym battle or raid, using a 1st gen poké, the battle goes all normal most of the times (sometimes have internet or server issues, but it's not because the poké). But, when I start the battle with a 2nd gen poké, or even when I switch in battle for a 2nd gen mon, the game start lagging, the movements and animations become slow, I can't dodge or attack in the normal speed and sometimes the game close. There's good pokémon in the 2nd gen for raids and gym battles, I don't want my game to close in a eventual legendary raid when using a tyranitar, heracross, umbreon, espeon, scizor...
Has anyone had this problem?
Does anyone know how to fix this problem?
Ah, sorry if there's something hard to understand, english is not my 1st language ;)