Silverlight AccordionItem hangs at switch

Question

I have an Accordion in a Silverlight interface which contains 2 items: one with a ListBox and the other with some TextBlocks and TextBoxes. The problem is that the ListBox contains a lot of items and every time the AccordionItems switch, the one containing the ListBox hangs and it takes a few seconds to load. I tried handling the selected and unselected events to counteract this effect, however this only reversed the behavior: now the closing of the ListBox AccordionItem hangs, instead of its opening.

Another unexpected behavior is that if I add a simple MessageBox announcing the selected/unselected events, the switch is made instantly after the MessageBox closes.

Did anyone encounter this effect? Do you have any workarounds for this?

Thanks!