Update the stats, run the query again and post the new actual exec plan please. Currently the incorrect row estimations are messing everything up, it's not going to be possible to provide useful advice until that glaring problem is fixed and the execution plan is sensible for the rows involved.

According to the plan you posted, the query returned 280093 rows in total. That 8.3 million was the actual rows on the index seek and the number across the plan before the aggregation.