This is a fairly good article on determining the source of powerview performance problems: http://blogs.msdn.com/b/psssql/archive/2013/07/29/tracking-down-power-view-performance-problems.aspx

This is a whitepaper on SSAS Tabular performance tuning: http://msdn.microsoft.com/en-us/library/dn393915.aspx

With how your cube is responding faster in excel I suspect your problem may be either a rendering one or an architecture (e.g. network io) one.

Good luck.