Yes, if you only put in 5 hours, you are not delivering at your full potential. You are very unlikely to be delivering "10x Developer" work. I have no idea what your company size is, but I would wager that the top developers at my workplace land 10x as much code as you (and solid, well-tested, and efficient code).
Yes, you are lazy. I don't know what else to call doing the bare minimum. I'm not passing judgement and I'm not angry at you for it. But everything you've described can be summarized as "I'm lazy and I get away with it."
If, on the other hand, you tell us that your manager has explicitly signed off on you working 5-10 hours a week, then would this not be laziness but rather a fantastic job arrangement.