DEFAULT Keyword Performance

I have a function which performs a query and returns a table. The one
parameter that can get passed in is a date which defaults to NULL.
There is an IF statement in the function that will set the paramter to
an actual date if null. If I call the function while passing in a date
the function comes back a second or 2 later. But if I pass in DEFAULT
to the function, the same query takes 8 minutes. See code below and
sample call below.

CREATE FUNCTION fCalculateProfitLossFromClearing (
@TradeDate DATETIME = NULL
)
RETURNS @t TABLE (
[TradeDate] DATETIME,
[Symbol] VARCHAR(15),
[Identity] VARCHAR(15),
[Exchange] VARCHAR(5),
[Account] VARCHAR(10),
[Value] DECIMAL(18, 6)
)
AS
BEGIN
-- Use previous trading date if none specified
IF @TradeDate IS NULL
SET @TradeDate = Supporting.dbo.GetPreviousTradeDate()

-- Make the query
INSERT @t
SELECT
@TradeDate,
tblTrade.[Symbol],
tblTrade.[Identity],
tblTrade.[Exchange],
tblTrade.[Account],
SUM((CASE tblTrade.[Side] WHEN 'B' THEN -ABS(tblTrade.[Quantity])
ELSE ABS(tblTrade.[Quantity]) END) * (tblPos.[ClosingPrice] -
tblTrade.[Price])) AS [Value]
FROM
Historical.dbo.ClearingTrade tblTrade
LEFT JOIN Historical.dbo.ClearingPosition tblPos ON (@TradeDate =
tblPos.[TradeDate] AND tblTrade.[Symbol] = tblPos.[Symbol] AND
tblTrade.[Identity] = tblPos.[Identity])
WHERE
([TradeTimestamp] >= @TradeDate AND [TradeTimestamp] < DATEADD(DAY,
1, @TradeDate))
GROUP BY tblTrade.[Symbol],tblTrade.[Identity],tblTrade.[Exchange],tblTrade.[Account]

RETURN
END

If I call the function as

SELECT * FROM fCalculateProfitLossFromClearing('09/25/2003')

it returns in 2 seconds.

If I call the function as

SELECT * FROM fCalculateProfitLossFromClearing(DEFAULT)

in which GetPreviousTradeDate() will set @TradeDate to 09/25/2003 it
returns in 8 minutes.