[ALLY] Why does panel use parent-face's pane?

Hi, just would like to ask about panel - it doesn't seem to use own pane, but uses pane of parent-face's field ... what's the point? I have large image put inside of panel, and moving it around by mouse is pretty OK. But once I put transparent face over it, it slowdowns drastically, no matter if I use saved-area: true ... Anyone? Thanks, -pekr-