I have a list with a little over 5000 items in it. I have filtered the data in a Weekly and Monthly view using calculated columns. This works fine until the list hits 5000 items. The filters stop working at this point for some reason. So even though the Weekly view would have less than 5000 items (more like 600 or 700) I still get a threshold limit message once the total for the entire list hits 5000. I have totaled columns in the weekly and monthly view to make sure they do not have more than i think. But when the list total is say 4999 the total for the Monthly view is 3600 and the Weekly view is 200-700, but when the list total hits 5000 both the views display the threshold limit error.
The 5000 limit is not on the number of items displayed, but on the number of items processed to get the result. To filter to find only one item in a list of 10,000 requires the scanning of all 10,000 items. The same applies to a sort. To sort and return the first 10 items requires scanning all 10,000 items to find the first 10.
Normal / simple columns can be indexed to work around the filter and sort issues. You cannot create an index on a Calculated Column.
One solution is to have a workflow that copies the Calculated column's data into a Single Line of Text column that has been indexed, then create your view using that column.