Many background apps
With each new App that you install the number of apps running in the background increases. As the number of background apps increase the phone might slow down.
The software isn’t fully integrated with the hardware
The Android software is an open source software that can run on different devices. Because the phone’s hardware was manufactured by a different manufacturer than that of the operating system many Android phones have poor hardware-software integration.
Apps became havier
As apps update themselves they can install new versions that are more resource consuming. This can result in slowing down the phone.
Android doesn’t have many restrictions
Apple restricts many of the operations and even installations that could slow down the device where as the open nature of the android system can force the phone to slow down dramatically if the wrong things were done.
Storage is almost full
Solid-state drives slow down when they are full. As the phone storage becomes full the device will slow down and the performance will be reduced.
No Solid-State Drive TRIM
Solid-sate drives slow over time because flash memory cells must be cleared before they can be written to again. The problem was fixed in Android 4.3 which introduced TRIM which clears the cells that used to contain files that were deleted.
UI thread isn’t processed as a priority
The Android operating system doesn’t give a high priority to the UI but instead it treats it as a normal thread. When any app uses much processing power the UI slows down.
Java is resource intensive
Android was coded using Java which is a resource intensive language that can easily consume up memory and so result in slowing down of the device.
Custom software added by manufactures
The Custom software added by each hardware manufacturer that uses the Android operating system contributes to slowing down the device. (See Why mobile phones lose value?)
Low quality apps
The Play store has many low quality unoptimized apps. Because Android doesn’t do the strict filtering of Apps that apple does the play store ends up with many apps that can slow the phone even more.
The NAND/NOR memory slows down
The NAND/NOR flash memory many devices use has a limited life cycle and it tends to slow down as the time passes.
Bad custom UI
Bad Custom UI such as the Samsung touchwiz also contributes to the slow down of the phone. If the UI has problems then the phone can easily slow down.
If any of the internal components became damaged then it might result in a slow phone. The older the phone the more likely this is to happen.
Overheating slows down the phone
Some operating systems slow down intentionally if the phone’s temperature rose. If the phone’s temperature went up the processing might slow down if the operating system has a function that does that. (See Why are Nokia phones stereotyped as being indestructible?)
Why are Nokia mobiles so strong and durable? Are Nokia phones unbreakable? Are Nokia smartphones still indestructible?