A QVD is a file format created by Qlik that is very efficient in size and indexing. So processing a QVD in an app will be much more efficient than process the raw data.
Creating a QVD is usually a 2 step process. Step 1 you extract the data from the source and store that in a QVD. Step 2 is you read in this QVD and do all of your transformation of the data. It is better to do as much of this outside of the app as you can.
Then you build your app with the Transformed QVD as your data source.
I'm sure there is a lot more detailed information on this site and on YouTube about this.