The OS Upgrades
If you bought your device a few years back, it may be running on Android KitKat or iOS 7. From time to time upgrades of these apps are released with a certain set of hardware specs which may not be compatible with your phone. This may affect the speed of your device.
Apps running in the background
Background apps can also cause your phone to slow down because the running apps are taking space. Hence, make it a habit to always close your apps if they are not in use.
Memory Degradation
Smartphones run on a flash memory. When the phone memory is full, your phone gets slower. Do not forget to intermittently clean your phone memory.
Higher expectations
Sometimes, after looking at several high-end smartphones, we expect our phones to perform better. You can't compare Galaxy S3 with Galaxy S8. So, we should learn to manage our expectations or simply buy another device.
App updates
Apps can be transformed from lightweight to heavyweight over time due to updates. Developers are constantly releasing updates and every new update brings new features, which ends up taking most part of your RAM and CPU.