Query Performance Problem

The following stored procedure is taking too long (in my opinion). The
problem seems to be the SUM line. When commented out the query takes a
second or two. When included the response time climbs to minute and a
half.

Is my code that inefficient or is SUM and ABS calls just that slow?
Any suggestions to spead this up?

Thanks,
- Jason

SET NOCOUNT ON

DECLARE @PriceTable TABLE (
[Symbol] VARCHAR(15),
[Identity] VARCHAR(15),
[Exchange] VARCHAR(5),
[ClosingPrice] DECIMAL(18, 6)
)

-- Use previous trading date if none specified
IF @TradeDate IS NULL
SET @TradeDate = Supporting.dbo.GetPreviousTradeDate()

-- Get closing prices from historical positions
INSERT INTO @PriceTable
SELECT
[Symbol],
[Identity],
[Exchange],
[ClosingPrice]
FROM
Historical.dbo.ClearingPosition
WHERE
[TradeDate] = CONVERT(NVARCHAR(10), @TradeDate, 101)

-- Query the historical position table
SELECT
tblTrade.[Symbol],
tblTrade.[Identity],
tblTrade.[Exchange],
tblTrade.[Account],
SUM((CASE tblTrade.[Side] WHEN 'B' THEN -ABS(tblTrade.[Quantity])
ELSE ABS(tblTrade.[Quantity]) END) * (tblPrice.[ClosingPrice] -
tblTrade.[Price])) AS [Value]
FROM
Historical.dbo.ClearingTrade tblTrade
LEFT JOIN @PriceTable tblPrice ON (tblTrade.[Symbol] =
tblPrice.[Symbol]AND tblTrade.[Identity] = tblPrice.[Identity])
WHERE
CONVERT(NVARCHAR(10), [TradeTimestamp], 101) = CONVERT(NVARCHAR(10),
@TradeDate, 101)
GROUP BY tblTrade.[Symbol],tblTrade.[Identity],tblTrade.[Exchange],tblTrade.[Account]