Yes, everything 'steals time,' but the biggest time stealers are those things which chemically alter your brain, such as alcohol, cannabis or other drugs. They alter the organs of perception in ways that reading a book or walking a dog do not.
Of course this is the fun of them. But also the downside.
Depends how you define 'biggest time stealers' - tv and video games are huge time stealers for many people. And the fact that drugs have a chemical effect on the brain does not mean they are necessarily worse than, say, watching some braindead tv show while stuffing your face with fast food, or sitting in a classroom for hours listening to a monotone voice regurgitate some dull bullshit you don't care about. Many people have had beneficial experiences with drugs, for example Steve Jobs with LSD.
Of course this is the fun of them. But also the downside.