Extracting from a discussion from sql.ru forum:
PowerPlay performs very bad when it comes to large and sparse (with big number of nulls and zeros) cross-tabs.
Modern OLAP engines are specially optimized for NonEmpty MDX clause, which is default report generation option.
PowerPlay was designed for fast cross-tab rendering and maximum amount of calculations on client machine. Nobody changed that, although it was promised. And nowadays dimensions with hundreds of thousands elements are very common and millions are not at all unique.
So if you now want to use Transformer — keep in mind that you should create most used reports in Report Studio, using filter clause to push suppression back to server.