Hi Patrick,
You can improve the performance of report rendering with following report item level changes.
1. If you are using Tablix report item and the CanGrow property given as true, then set CanGrow property as false for all textboxes which has static text also possible items which will not exceed design height.
2. As said above point you can set CanGrow property as false for Tablix header rows and any static cells.
3. Remove unused datasources and datasets in a report.
Regards,
Vinoth S.