another slow device question topic

I have read pretty much forums and other websites to find a solution for my problem, but did not find any so far.

I am happy with my Galaxy S2 for more than three years now.

The last 1,5 or 2 years I have installed and tried a lot of custom roms, Ice Creams Sandwiches, KitKats and now also Lollipop (Omni and CM).

The last half year I have the next problem.

After installing a new rom and installing the apps I would like to use, the phone is nice smooth and fast, with a nice fluid UI.
But after some time ( after 1 or 2 weeks), the phone realy becomes suddenly very, very slow....opening apps takes a lot of time, often 5 seconds or even much more :(

Clearing cache, restart, clearing dalvik, nothing helps....the only thing that works is a complete fresh install.
There is enough diskspace and free space. I could even install quit some extra apps if I would like to.

This is the way I install a new rom:

- factory reset
- clear dalvik, system, data
- sometimes I reformat the internal storage (which is best: fat32, EXfat?)
- flash kernel wipe
- install rom, gapps and mostly superuser
- reboot the phone, do the settings
- install all wished apps
- sometimes I do a dalvik wipe and reboot again.

The phone is now nice smooth and fast.
The strange thing is, after these steps, I do not install anymore apps or widgets anymore. But still, after about a week, the smoothness is over and all that left is a lot of lag......

Does anybody know what causes this and how this can be solved?

Is it possible, that the flasdrive has become corrupted or bad, after this three years and if yes, can this be tested?
Any ideas would be great!

Best Regards,
Branko