On some machines I've tested on, having a lot of droppables makes dragging really, really jerky and slow. I added an option (quiet) to draggables that, if true, will only activate droppables on drag -end- (rather than on updateDrag). It helped performance considerably for my application.
It might be nice for everyone to be able to turn off activation notification for a draggable if they're not going to need it (since it cuts waaay down on all the Position.within calls, which can slow things down).
