SELECT Statement With Multiple Criteria That Returns The Criterion Matched

Hello:

I need assistance writing a SELECT statement.  I need data from a table that matches one (or more) of multiple criteria, and I need to know which of those criteria it matched.  For instance, looking at the Orders table in the Northwind database, I might want all the rows with an OrderDate after Jan 1, 1997 and all the rows with a ShippedDate after June 1, 1997.  Depending on which of those criteria the row matches, it should include a field stating whether it is in the result set because of its OrderDate, or its ShippedDate.  One way of doing this that I've already tried is:

SELECT 'OrderDate' AS [ChosenReason], Orders.*
FROM Orders
WHERE OrderDate > '1-1-1997'
UNION
SELECT 'ShippedDate' AS [ChosenReason], Orders.*
FROM Orders
WHERE ShippedDate > '6-1-1997'

In my application, scanning a table with thousands of records for five sets of criteria takes a few seconds to run, which is not acceptable to my boss.  Is there a better way of doing this than with the UNION operator?

Thank you