Infested blocks don't have an effective tool, using a pickaxe does not effect the time it takes to break. With most pickaxes it takes longer to break an infested block then it does to break the normal variant.

Steps to reproduce:

1. Place down two of the same infested block
2. Break one with pickaxe
3. Break the other with your hand

• Notice how it takes the same amount of time to break it with a pickaxe then with your hand

Note: This is probably the case because infested blocks used to be instamine, so it didn't need an effective tool. But when they changed it to not be instamine, they forgot to give it the effective tool.