When you use Data Analysis Expression (DAX) functions such as COUNTA, AVERAGEA, COUNTBLANK among other functions to perform some operations on a String or Boolean column, you may experience poor performance on Microsoft SQL Server 2014 Analysis Services (SSAS).
The issue was first fixed in the following cumulative update of SQL Server.
Cumulative Update 2 for SQL Server 2014 /en-us/help/2967546
Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server:
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.