There is a known limitation on SQL Server 2012 and 2014 SSAS tabular model. If you have a table with multiple partitions, when you process these partitions, only one partition can be processed in the same table. Partitions from different tables can be process in parallel.
SQL Server 2016 CTP2 has an improvement to allow parallel processing of partitions from the same table. CTP2 can be downloaded from this link http://www.microsoft.com/en-us/server-cloud/products/sql-server-2016/?WT.srch=1&WT.mc_id=SEM_BING_USEvergreenSearch_Cloud_Cloud|Bing|SEM|DI|SQL%20Server|Brand|US_MSFT
Below is what I found when testing the CTP2 build.
SQL 2016 CTP2 installation has PowerPivot as part of the SSAS Server Mode selection. This makes installation more streamlined.
The SSMS processing diaglog hits a bug when connecting to this instance. This is something that should be fixed in the next CTP build.
I am able to confirm the parallel processing is in the SSAS engine. However I have to manually modify the XMLA command to include the <Parallel> clause. Otherwise, if I just use the command generated from the SQL Server Data Tool (SSDT), it is still a serial processing. SSDT should be updated to add the parallel option. If I process the whole table, the partitions are processed in parallel by default.
This improvement should be an exciting news for any SSAS tabular model customer. This should bring an improvement to tabular model processing performance.