Hey all, after my Galaxy M21 updated to OneUI 2.5 I noticed a stutter or lag in recent apps transitions and on the UI in general.
It came out of nowhere and it went away after I restarted my phone, however it got to a point that not even a reboot would help.
I tried wiping cache and it worked but not for long. Desperate, after finding no information online, I decided to disable all animations under developer settings 
Fortunately my endless will to troubleshoot (lol) led me to a Samsung's forum post where a Galaxy S20 owner was experiencing the same issues and suggested to re-enable any disabled apps for the lag to disappear.
I went right away to try it out and that seems to have worked as my phone feels buttery smooth when switching apps and I haven't had the stutter/lag come back like in a full day!
Oddly enough I had disabled just 3 apps (Game Launcher, Netflix and OneDrive). I figured out enabling just the launcher app should do the trick as it's part of the Samsung environment/system, and thankfully it did.
I really hope this helps anyone out there experiencing lag on their phone's UI. I tried other stuff that didn't last for long but this, does put a smile on my face 
I have the same problem with my Galaxy M31s. I am solving it via restarting but this means 2 or 3 restart a day for me! I tried your solutoin and I will observe it. I am going to update my answer if I face the problem again. I hope I will not.
Edit: Unfortunately, it started to do it again
