Nigel Foulkes-Nock explains why that first query after restarting SSAS can be slow:
When the SQL Server Analysis Services (SSAS) Tabular Service is started, it can take a long time before it is ready to be queried. This can cause delays to Service, not to mention confusion.
This Blog Post will explain what is happening during this time and a method that can be used to improve. It’s worth mentioning that the SSAS Tabular Databases that this has been used on are quite large (> 100Gb).
Click through for the answer, as well as a technique to warm up those servers so an end user doesn’t wind up being the one to pay for this wait.