For web, you'll find that performance with version 7 is much snappier because there are no activex / java downloads, and no sql generation, and no database connections on the web server.
Intelligence server has been revamped to house all application logic including sql generation. But they've also improved the performance with caching for all user operations (including loading elements for filters, reports etc). Overall, the performance is on par, if not slightly better with intelligence server 6 vs 7.
Nick