That makes sense, and I think you may be right about large scrolling levels (with many objects bouncing off the background) being a performance bottleneck in XNA.
For the record I have not noticed any serious problems with Fastloops slowing things down, as long as I keep them all in separate deactivated event groups and only run the loops when needed. The only one that really slowed things down with my game was a Fastloop-based wall crawling movement constantly looping through 50+ actives. Using Fastloops for things like finding which instance of an enemy was destroyed and destroying all its child sprites at the same time works brilliantly.
