It was painful. He had learned about it in the Defence class. Mad-Eye had demonstrated it. But he had never expected it to be this painful.
When Voldemort had cast it on him he barely even had the chance to flinch. Harry was quiet a stubborn lad. He didn't want Voldemort to have the satisfaction of hearing him scream. So he held himself shut. Not screaming. Biting his tongue.
It didn't last. By the second crucio he wounded his tongue. By the third he had screamed.