You need to look at the CurrentDateMember function, see Diethards blog here for a comprehensive explanation:
http://diethardsteiner.blogspot.co.uk/2009/10/current-date-function-on-mondrian.html
As for performance - the only solution is to turn on the various mondrian logs, and check that firstly there is no slow SQL - if there is then fix that in the database first - chances are a missing index or something.
Finally if it's still slow then post the full mdx. You dont have 2 separate date sets do you?