I can't say for sure if it's normal because I don't do iPad testing, but at the very least you can expect much shorter battery life than what Apple quotes on their
website (
up to 10 hours). Reasons being:
Apple tested with a movie purchased from the iTunes Store. It's likely the iPad has an easier time playing that movie format than something from Youtube.
Apple's test movie was 2 hours 23 minutes long, played on repeat. At the very least after the first play through the movie would have been cached locally, while if you were playing a single long video it was streaming new data for the whole duration.
Apple tested with auto-brightness off and brightness set to 50%, while you were using 100% brightness the whole time.