Slow Charging at 90% with Optimised Battery Charging turned OFF

September 2022

• iPhone 12 Pro Max 256 GB
• IOS 16.0
• 20W UBS-C Power Adapter (Apple)
• Optimised Battery Charging - OFF

Charging time slows down when it reaches 90% charged even with the Optimised Battery Charging turned off, I have a shortcut that runs a notification when my phone is charged as I charge it when I am awake and near the phone, but I have noticed it will fast charge until 90% every time and then slow down considerably.

Why is this happening?