Optimize Query

Hello, this query executes really slowly and will get slower as more records are inserted:

Select      SUM(
Case DisplayFormat
WHEN 1 THEN 1
ELSE 0
END),
count(Distinct DomainID)
From Displayed
Where(SnapID = 1000)
and dDisplay > '17-JUN-2008'

Any Ideas on how to improve query  speed? I think the issue lies with the Case DisplayFormat. Does this run through every record in the table or do the where restrictions also apply to the case statment?

Thanks