>>35084382IVs are more significant to fast attacks than to charge attacks.
+1 to every fast attack adds up more than +5 to a charge attack.
Here's an example, Rayquaza with Dragon Tail vs Rayquaza
>Any level 30 Rayquaza will do 19 damage>15 IV means level 31 can do 20, level 37.5 can do 21>9 IV cannot reach 21 damage, but no legendary goes under 10 IVs anyway 