slow down OS13.5.1
Over the past month June 2020 my updated iPhone 6s+ to OS13.5.1 has slowed down opening apps such as Mail, Photo on iPhone and iPad.
iPad mini, iOS 9
Before updating to the latest iOS 13.5.1 as told on your post what was your earlier iOS version. Then you can look into the iOS changes that has happened between the earlier version on your iPhone and this new version that you have updated now.
This will give you a broader idea of the issue, if it is directly linked to the iOS version or is triggered by something else.
Because iOS 13.5.1 was only for security update, you may probably be carrying a legacy issue from previous iOS versions.