Hi Adina,
In general, groupings slow reports down.
With filters, the more you filter, the faster the report should load (that's my impression at least).
For calendars....the only way I can get them to perform reasonably is if using the Gantt view.
Regards, David
From my experience there are browser differences, Chrome is a lot lot faster than Internet Explorer. But the main tip is not to use Gantt charts as they are pretty slow.
I would also place filters on the view, seems to be quicker than at report level.
When you filter on any text (contains, for example) you will be causing the system to scan all the text in every item on the system. Limit your filters to indexed fields and you'll be better off. These would be things like portfolio , program , status, etc. Dates are also generally ok, but when you get into custom data, the system may not have it all indexed - use these sparingly.