My Apple device start fast charging and slows down (Macbook charging from 100W to 60W)

Both HyperJuice and Apple Products are built with protection, safety, and longevity of the batteries in mind. 

When the Macbook is drained, it will accept full 100W Charging speed then as its being charged passing about 60% or more, MacBook will start charging at a lower speed, which is why you may see charging start at 100W and slow down to 60W. 

The HyperJuice will also charge at a slower speed as it drains below 50% to help protect itself for long term use. 

* Resources

https://www.apple.com/batteries/why-lithium-ion/

https://www.apple.com/batteries/maximizing-performance/