Performance of Report

I have a report that looks something like this.  I am only showing two charts but it contains 14 charts very similar to these two:

The queries or datasets behind these charts are pretty much the same.  For example:

SELECT CollectionDateTime, CounterName, CounterValue 
FROM [CMSRepository].[dbo].[SysDmOsPerformanceCounters]
WHERE CounterName = 'Page life expectancy'
AND ServerName=@ServerName
AND CONVERT(DATE, CollectionDateTime) >= @Start 
AND CONVERT(DATE, CollectionDateTime) <= @End;

SELECT CollectionDateTime, CounterName, CounterValue 
FROM [CMSRepository].[dbo].[SysDmOsPerformanceCounters]
WHERE CounterName = 'Batch Requests/sec'
AND ServerName=@ServerName
AND CONVERT(DATE, CollectionDateTime) >= @Start 
AND CONVERT(DATE, CollectionDateTime) <= @End;

When I run the report it takes about 10 minutes.  I don't understand why it is so slow.  I tried putting in an index and when I run the query in Studio Management it is faster but not in Report Services.  Is there something that I need to change on the report itself or on the installation of Report Services?

lcerni