Multiprovider Issue

Hi Experts,

I have created a Multiprovider (union of 5 cubes) and generated 3 querys. One Query is accessing data from three cubes? If i execute the Query it is going the read all the 5 cubes data and based upon the Query definition it is going to retrive the data from olap. How can improve the performance of this query? Can any one explain it in brief?

Thanks in Advance

Vara