APPS, APPS, APPS: Lots of APPS in older phones slows down the performance.
There are generally too many APPS running in the background: you can explore the settings tab to close the unwanted APPS running in the background. In few mobiles, you will find an ultra-saver mode that will help stop running unwanted APPS in the background.
Lack of Storage Space: another reason for slowness is no memory space or less free memory space available in your phones. Make sure to transfer or delete your Photos and videos from time to time.
Cellular network Updates: cellphone networks continuously keep tweaking their networks to make them faster but the updates are generally available only to the newer phones. Older phones will not get these benefits.
Perception: with time our expectations increases from our older smartphones as the newer phones start influencing us. Thus, we start considering that our older phones have started performing slow.