Same problem for me, I have just do a clean flash to see if there was related to a specific app. I my case sometimes after getting slower and slower the phone reboots itself.
This did not happen with stock kernel or EucOs kernel.
I tried the low memory killer with kernel adiutor. Checked out all the different profiles there, but it didn't help at all. After 10 hours, the phone was unusable until a reboot.
I wonder if the problem is hardware-related. Otherwise there should be much more people experiencing this slowing behaviour.