boost::function2's copy constructor expensive - why?

Hi! Today's profiling of my code (which massively uses boost::function to collect formulas) showed that boost::function2' copy constructor is called zillions of times. OK, I disabled inlining to get some more details, but this I do not understand. The functors are created, stored in an array and do not get copied again, but rather referenced there. Before I dig in that code: where do those copies come from? Is that the normal behaviour triggered ny operator() or is that a hint that I use them the wrong way? Any experience? At present the boost::function overhead kills my application. Tuning tipps? best regards, Markus