If you use SSAS Tabular, this is a very important news!
Microsoft released a very important update for Analysis Services 2012 that provides performance improvements to pivot tables connected to an Analysis Services Tabular model: it is SQL Server 2012 SP2 Cumulative Update 8.
Microsoft discussed some of these improvements in this blog post: Performance problems on high cardinality column in tabular model
UPDATE 2015-09-22: I fixed this post in the following part.
In a previous version of this post, I wrongly reported that this fix was fixing the unnatural hierarchies problem, too! This is described in the article Natural Hierarchies in Power Pivot and Tabular. In reality, only Power Pivot for Excel 2016 and SQL Server Analysis Services 2016 fixed the issue, which is still present in previous versions of Analysis Services (2012/2014) and Power Pivot for Excel (2010/2013).