Game rendered unplayable due to crashes

As much as I want to play this game, and take part in raid battles... I cant. Every time I boot up the app, it lags like nuts (almost to the point of freezing) and crashes within several minutes. Please find some way of fixing this problem.

From,
An iPhone 6 Pokemon Go player.

Haruka Kaze about Pokémon GO, v1.39.1