Impromptu is more geared to showing detailed transactions, whereas PP is more of a summary of the same transactions. From PP can can drill back to the orginal Impromptu report, so you can have the best of both worlds. Once a cube is made performance in PP is extremly fast.
This is an over similifaction, and is often possible to the same report in both programs.
Performance tips are all over this forum and the pdf that came with the software. To very quickly sum them up: filter/ sort at the server level, make sure that filters/ sorts are done on indexed fields from the database, make sure that your connection to the data base is set up correctly, if you are going to use PP, don't bother using sorts in Impromptu, run large reports or cubes at night using a macro.
The gurus here will jump in with several more tips.
Hope this starts you off in the right direction.
Bruce