Distinct Count calculations in DAX are very fast compared to other engines. But when you have a large number of values, performance might be different depending on the query you write. This paper investigates different techniques that produce the same result with different performance, showing that you need to make
Understanding DAX query plans is a long process. This paper explains how to capture and read a DAX query plan, helping you to find possible optimizations of your DAX queries.
SQL 2012 gives you two different xVelocity engines, with different capabilities and different scenarios of implementation, the paper will help you choose the right one or, at least, raise your curiosity about performing more tests on your specific scenario.