First of all, if we've received some Android OS updates, they may not be as nicely optimized for our devices. Even our carriers or manufacturers may have added additional bloatware apps in an update. These may slow our systems down.
On the other hand, in general, we should have installed a lot of apps that run in the background. We also have a large amount of widgets on our home screens. All of these may consume CPU resources, fill up RAM, and slow down our devices.
In addition, apps, their continuous updates, and their cache files can consume quite a bit of storage space. If the internal drive of our devices is almost full, writing to such file system may be very slow.