I would set Cache options for this report.
From the Folder view, click on the Open Menu button next to the report and choose Manage, then Processing. Choose the "Cache a temporary copy ..." option that best suits your scenario.
Personally I dont like the "clicking on plus" UI. You may find that the HTML rendering effort is also slow once the dataset is cached.
Instead I prefer to set Bookmarks which result in a Report Map (hyperlinks). The UI is cleaner and faster.