>>>Did you try having an index on the group by expression?
>>>
>>>Also, you do really need to convert to char? why not grouping by YEAR(Invoice.ModDate) directly? (and having an index on it too might help)
>>
>>In SQL Server you can not create an index on expression. Only if it's a computed column.
>
>Oh, yes, little detail I forgot... sorry Michel! But still you do not need the convert in my opinion, although I already proved I could be wrong!

Yes, no need to convert in the GROUP BY expression. I suspect there is a reason to convert in SELECT fields list.