8.1 oreo – Android devices getting slower over time

At work we have a bunch of Casio handheld android terminals (DT-X400). They do not have any connectivity to the internet (so no updates), almost all apps are disabled except for those we use for work. We used them for about a year, and recently got a few more.

When we started using the new ones, we discovered that they perform a way, way better:

  • no lag;
  • navigating within applications is fast and smooth;

while the others feels sluggish:

  • it takes few minutes to boot up;
  • often it lags while doing simple actions, like scrolling on small lists;
  • opening apps takes a long time (> 30 seconds for calculator) – when you click on the app icon it looks like it freezes, then after a while screen goes black and after few more seconds app opens up.

The new ones performs a lot better when doing same tasks.
They are all the same model, hardware is the same, drivers versions on every device are also the same. All of these terminals runs Android 8.1.
I tried clearing memory, disabling all the apps I can, later did a factory reset, but performance did not improved.

So my questions are:

  • What can cause such slowdowns of android devices?
  • Is there any way to improve performance, without getting root access?