Exits from different applications, The phone is slowed down,
What is the current reading for Maximum Capacity on the battery?
Settings > Battery > Battery Health & Charging > Maximum Capacity
The screen often does not detect
This can also be a battery issue, or another issue known as "ghost touch", which is an indication that the screen is beginning to fail.
The iCloud storage is full.
Do you have a current or recent backup of your phone? The reason that we ask is if you restore or reinstall the iOS , this will erase all of the data on the phone, so you will need a complete recent backup to set up the phone again. If you don't have a current backup, the phone will look like it did when it was brand new, and you will have to manually add apps, music, videos, etc.
Sorry for all the questions, but I think you would agree that it would not make much sense to erase everything on the phone if it does not appear that reinstalling the iOS would help. As it stands, I do not have much confidence that reinstalling the iOS will help, since this appears to be more of a "try this" than a solution.