Very slow !

I'm trying to directly create an online cube with the Analysis Manager with a data source pointing to a cube file supplied by a customer. In principle it works, using OLAP Services 8.0 as the data source, but I soon as I start to add in more than five dimensions, the processing time just explodes. All I want to do is share a cube file file I receive on a daily basis, with around 300 people over the network. Any ideas ?