Were you using the phone each day to battery 0% straight? Did you consider getting the battery replaced? Those performance issues are completely explainable b

Were you using the phone each day to battery 0% straight? Did you consider getting the battery replaced? Those performance issues are completely explainable by the battery age + performance throttling for aged batteries, if you were a 100->0% without any charge during the day kind of user over the course of 1.5 years.