He likes to complain that the fourteenth century was bloody boring. He even tells himself that's how he feels, though he doesn't mention it around other demons, who like to reminisce about the Plague and the subsequent collapse of society in much the same way that elderly humans sit around and talk about how things were cheaper and better-made in their youth. When Aziraphale questions him about it - I thought that would have been your sort of thing, old boy - he shrugs it off with a sneer, and a comment about the lack of creature comforts during the worst epidemic the world has ever seen.
The truth is that it was horrible. The truth is that there were bodies everywhere, and the stench of burning corpses, and Crowley, who hadn't seen the Pit in five thousand years, was vividly reminded of his reasons for staying on Earth.
He'd been afraid then that the world was ending, that it was all just over and no one had told him, and he hadn't known then why the thought made him almost sick with fear. He only figured it out centuries later, while Armageddon threatened and Aziraphale's books burned around him.