Maybe have a look at
(the document is about QV server, but I think the basic principles are still valid)
In general, there is no need to free memory for QV if the memory is not requested by another app, so there is nothing really wrong with keeping some information still in memory to allow for faster re-accessing the information when re-using the application.
This largely remains true in Qlik Sense with one exception, inside the Engine settings there is a value for how long an app will stay in memory (Editing an engine ‒ Qlik Sense) called App cache time. This value + the above document should get you where you need to go.