This Datapack adds weight, sounds boring and your right. But Ill show you how it works anyway.
If you wear leather, you will not be weighed down.
(Most helmets will not slow you down, or will only slow you when stacked with other armor)
Chainmail, You will have 0 - 2
Iron, You will have 2 - 3 Slowness
Gold - Diamond(I think netherite too) is 3 - 4 Slowness
Gl and have fun