Player weaker than usual but without potion effects.

I want to create something similar to the "Better than Wolves" mod. 

I'm new to MCreator and don't know if it's even possible with the software.

If it is, can someone provide me with a way or some instructions on how would i make the player weaker?

(by weak i mean something like slower block breaking)

[2020.2 MCreator // 1.12.2 Minecraft Version]