Receiving the summoning makes Bellatrix drop everything she's doing to prepare—except that two minutes into this rush of excitement, it sinks in that the Dark Lord will not be there. That slows her enthusiasm, but she's still very, very curious to see what their supposed superiors will have to say. If they will take action, or continue to sit on what Lord Voldemort built.
She arrives about ten minutes early in pristine robes and her mask, looking every bit the high-ranking Death Eater that she is.