SQL Server 2008 :: Slow Row Number Over Partition

I have a simple table with 4 columns

(idAuxiliarPF(BIGINT+PK), pf(BIGINT+FK), Data(DateTime), Descr(NVARCHAR))that has aprox. 50k rows.

I need to create a partition of the data to join to another table, the query that i have:

SELECT
ROW_NUMBER() OVER (PARTITION BY pf ORDER BY Data DESC, idAuxiliarPF DESC) AS RN,
pf,
Data,
Descr
FROM dbo.PFAuxiliar
WHERE Data <= GETDATE()This query takes around 40 seconds to return the results

If i remove the Descr column, the query it takes no time.

SELECT
ROW_NUMBER() OVER (PARTITION BY pf ORDER BY Data DESC, idAuxiliarPF DESC) AS RN,
pf,
Data
FROM dbo.PFAuxiliar
WHERE Data <= GETDATE()I have two indexes, Clustered (idAuxiliarPF), NONClustered(pf).

How can i improve the performance of this query?