Symptoms
Assume that you are using Microsoft SQL Server 2012 Analysis Service (SSAS 2012) or SQL Server 2014 Analysis Service (SSAS 2014). When you run a Multidimensional Expressions (MDX) query against a large database that has a sub-select of different granularities, the query may take a much longer time than the single granularity under the same logic.
Resolution
The issue was first fixed in the following cumulative updates for SQL Server:
Cumulative Update 2 for SQL Server 2014 SP1
Cumulative Update 8 for SQL Server 2014
Cumulative Update 6 for SQL Server 2012 SP2
Cumulative Update 2 for SQL Server 2014 SP1
Cumulative Update 8 for SQL Server 2014
Cumulative Update 6 for SQL Server 2012 SP2
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.