There are 2 major problem areas:
1. Slow performance
2. Slicing fails
For both areas start with analysing the log-file, this should give you ideas on where time is spent and where slicing fails.
If you use automatic slicing you can go for explicit slicing using the "s" parameter.
Usually lots of time is spend analysing the cube for automatic slicing, switching to explicit can save considerable amounts of time. It could also be the solution if automatic slicing fails, search the logfile for more suitable characteristics(number of members) to slice on.