I think it's fairly common, or more so than you may think.
I also believe, knowing 2 people who were like this, a doctor/whoever will simply advice you to take the food away and not give them anymore. They will soon learn if they want to eat, they should eat faster.
It's harsh, but providing adequate time, like if he always takes 4 hours, let him only have 2 hours to eat. Then take it away. Then maybe 1hour 45mins etc...
Not sure who exactly you see about this, start with a GP who can refer you I guess. But that's the advice you will likely get. Of course, get professional advice. They will give you a suitable time limit and schedule!
So your dad is basically doing what he should be, although perhaps jumping down to 1 hour isn't so good straight away. But I'm not a doctor/whatever.
Are there any other distractions while eating? Such as a TV, or a computer screen he watches? If so, those should probably be removed so the focus is food, not other things.
As for why, he just is. He can easily learn to eat faster with time.