Hi Utkarsh,
Thanks for contacting Syncfusion support.
We have already working on the performance improvement in ReportViewer while loading the huge data and changes will be available in our Essential studio volume 1 SP 2, 2018 release which is tentatively scheduled to be released in the month of April. Also the following report level changes also improves the performance of report rendering in report server.
1. We are calculating height for textbox report item with CanGrow property as true so, you can set CanGrow property as false for all textboxes with 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,
Yuvaraj D