In-Memory is nothing but RAM.
In Qlikview entire data lies with you QVW file itself readily available and it need not go to data sources every time.
When you open any Qlikview report entire application data would be in Server RAM.So data is readily available and response time is seconds.That means user never waits to see the data.
Defending on your selections SQL (Query) will generated and sent to database to fetch the data. Depending on how much data you need the execution time varies from minutes to hours.User needs wait till data fetched from source and render.