When tuning performance there are certain steps you should follow to allow you to measure the effect of any changes you make to your cube, its calculations or the query you're running:
Always test your queries in an environment that is identical to your production environment, wherever possible. Otherwise, ensure that the size of the cube and the server hardware you're running on is at least comparable, and running the same build of Analysis Services.
Make sure that no-one else has access to the server you're running your tests on. You won't get reliable results if someone else starts running queries at the same time as you.
Make sure that the queries you're testing with are equivalent to the ones that your users want to have tuned. As we'll see, you can use Profiler to capture the exact queries your users are running against the cube.
Whenever you test a query, run it twice; first on a cold cache, and then on a warm cache. Make sure you keep a note of the time...