Prometheus Bound is about a guy who gets chained to a rock in the Scythian wilderness, who then gets plunged into a deep hole underneath the earth, and who will one day (so we are told) be lifted out of that hole only to have a vulture come and feast on his liver every day for centuries. So, yeah, there's suffering. And then there's Io, who gets transformed into a cow and driven over the face of the earth, constantly stung by a gadfly and spurred on by the ghost of the herdsman Argos. So, who has it worse? Prometheus, who knew exactly what he was getting into and has a goal; or Io, who's just had a run of bad luck? Is the mental suffering of not knowing your fate worse than the physical suffering of torture?
Io suffers more than Prometheus because Prometheus knows that he is suffering for a good cause, while Io is being punished for no fault of her own.
The play portrays gods as suffering more than mortals. Because Prometheus can't die, his torment will last much longer than any human could possibly endure.