Why is my battery charging so slow?

The power of the adapter does not decrease because it is directly relational to the incoming energy source (Electricity) and then converts it to the volts and amps that iPhone can use to charge its battery. Your issue is not the adapter or the cable.

The problem with your charging is the Battery. Battery Health Maximum Capacity has dropped below 80%. You are at 75%. At this point you are nearing the end of your battery lifespan. Apple recommends that if your battery health reaches 80% or below then replace the battery. I therefore recommend that you have your battery examined at an authorized repair center and get it replaced after confirmation. Thanks.

Axel F.

Two Questions for you -

1. Which charger and cable are you using?
2. What percentage is showing in your battery health? (Open Settings➔Battery➔Batter Health and then note down the Maximum Capacity Number)

Axel F.