Fast query runs slow in SSRS

Thanks for the suggestions provided here. We have found a solution and it did turn out to be related to the parameters. SQL Server was producing a convoluted execution plan when executed from the SSRS report due to 'parameter sniffing'. The workaround was to declare variables inside of the stored procedure and assign the incoming parameters to the variables. Then the query used the variables rather than the parameters. This caused the query to perform consistently whether called from SQL Server Manager or through the SSRS report.

Add this to the end of your proc: option(recompile)

This will make the report run almost as fast as the stored procedure

I will add that I had the same problem with a non-stored procedure query - just a plain select statement. To fix it, I declared a variable within the dataset SQL statement and set it equal to the SSRS parameter.

What an annoying workaround! Still, thank you all for getting me close to the answer!