The IItemDestroySpeed function can be added to an Item and will determine the item's block breaking speed.

It might be required for you to import the package if you encounter any issues, so better be safe than sorry and add the import.
import mods.contenttweaker.IItemDestroySpeed;

The IItemDestroySpeed is a function with the following parameters:

• IMutableItemStack mutableItemStack → The item.
• ICTBlockState blockState → The state of the block that is being mined.

The function needs to return a float value.