"Time flies..." as Spiegelman would put it, describing different events in Vladek's life (which includes his death) and his own. The "time flying" is more apparent as flies swarm over the troubled writer/artist finishing up a page on the drawing desk.
The corposes surrounding Spiegelman indicate he has tremendous guilt weighing upon him. The deaths of more than six million Jews weigh on him because his father's horrendous experience weighs on him. Spiegelman had to contend with his father during his childhood, even going far to assume his parents groaning at night was "normal" because he assumed everyone's parents did it. Spiegelman had to contend with his parents' obsession over his older brother who did not survive the Holocaust.
Spiegelman is ultimately weighed, ironically, by the critical and commercial success of Maus. He feels he acquired acclaim and money from the horror and death that befell upon the Jewish people by the Nazis. Spiegelman wears the mouse mask because he is a highly recognized figure in the world of comics. Does this make him feel any better? "Lately, I've been feeling depressed." he responds. Just like he cannot escape his troubled heritage and parent's haunted past, Spiegelman cannot escape fame and countless offers to commercialize his creation.