Performance tuning means simply reducing the loading time of the charts and Dashboard loading time. Performance tuning in Qlikview is done at various stages, ie.,in front end and backend of the qlikview application.
Do all the required calculations in the script itself.
Load only required data, for example if you are using only last 5 years data filter it out when you are loading data.
Load only required columns and tables so that Qlikview file size would be reduced.
If you are loading same tables in multiple dashboards then for the instance load from database and store it in QVDs and in other Dashboards load from QVD.
Use Set Analysis as much as possible.
Don't display all charts in a single sheet or provide buttons to show and hide the charts.
Don't display huge data in Tables, restrict user to do some selections so that the data would be reduced and load the table faster.
Limit the calculations in front end and move it to scripting.