I think it is based on the usage time of your phone, I had the same issue but on my note 9, after nearly 4 years my phone started lagging and the UI isn't as smooth as it used to be, I have tried multiple solutions like restarting and reset the factory and deleted some apps but still this kind of lags still happening especially after opening multiple apps at once, turned out that it is due hardware expiry and all these solutions just temporary at best.
buy another phone bro that is the only solution