How much do stats affect pokemon?

I want to know how much stats affect a pokemon.
Some of the questions I have is how much does the lost of increase or decrease in 20 points in stats affect a pokemon, for example Zacian before and after the nerf and others like how garchomp losing 10 speed is so detrimental.
On the side note, how much increase or decrease of a stat is actually meaningful to a pokemon: losing or gaining 2 points, 10, 12, or like 20?