Stored Procedure Works But Very Slow (was Optimization)

I have a big table A_newHistory (more than 2 million rows) with primary key fund_id + date_price . This table has to be updated every 2 hours from XML.
Every row in XML must be inserted or updated (if current id and date already exist in the table) in the A_newHistory.
The following procedure works but very slow...
How can I optimize that?

================================================== =======
CREATE PROCEDURE spSaveFundsAdjustedClose
@XML ntext
AS
DECLARE @fund_id int
DECLARE @date_price datetime
DECLARE @adj_closed float
DECLARE @XMLDoc int

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION

EXEC sp_xml_preparedocument @XMLDoc OUTPUT, @XML

DECLARE MutualFunds_Cursor CURSOR LOCAL FAST_FORWARD READ_ONLY FOR
SELECT *
FROM OPENXML (@XMLDoc , '/xml/a', 1)
WITH ([id] INT,[date] datetime, price float)
OPEN MutualFunds_Cursor
FETCH NEXT FROM MutualFunds_Cursor
INTO @fund_id, @date_price, @adj_closed

WHILE @@FETCH_STATUS = 0
BEGIN
IF EXISTS (SELECT * FROM A_newHistory
WHERE id_fund = @fund_id AND date_price = @date_price)
BEGIN
UPDATE A_newHistory
SET adj_close = @adj_closed
WHERE id_fund = @fund_id AND date_price = @date_price
END
ELSE
BEGIN
INSERT INTO A_newHistory
VALUES(@fund_id, @date_price, @adj_closed)
END

IF @@Error <> 0
BEGIN
ROLLBACK TRANSACTION
SELECT -1
RETURN
END

FETCH NEXT FROM MutualFunds_Cursor
INTO @fund_id, @date_price, @adj_closed
END

EXEC sp_xml_removedocument @XMLDoc
CLOSE MutualFunds_Cursor
DEALLOCATE MutualFunds_Cursor

COMMIT TRANSACTION
SELECT 0
GO
================================================== =======