How does the session timeout affect the system performance.
QlikView all versions
With default settings user session gets closed after 30 minutes and all selections and filters are lost.
The main effect of changing the session timeout will be that documents will stay in memory longer. A user session is not closed when the browser is closed. It is only closed after this timeout is reached OR if the user uses the close button in the document. The document timeout (the time that the document stays uploaded in memory) only starts to count down after all sessions are closed. But if the document is used often anyway it will be in memory either way. There is also a slight added load on the server with more sessions open. There is also the option of using session recovery, see article Documentation of Allow Session Recovery and restoring QlikView Full Browser Sessions
For an overview of timeout settings in QlikView see Timeout Values Related To QlikView Deployment