Its not so much that he's a dragon that perceives time differently. Its that he's a species with an incredibly long life span. Let's look at it this way: Summer as a 10 yr old vs Summer as 20 yr old? Which one felt longer? Now how would a 40 year old feel about Summer? keep upping the time and certain things become quicker, add in to it immortality (of a sorts a celestial dragon possesses) and years can become days or even hours. That's why its fresher on his mind. That and he's a psycho that wants them to suffer.