I definitely agree with you. Having an extra hour as buffer is helpful but if you do something important during your productive hours, you can finish even faster. I read somewhere though that time management is a misnomer, since we can't really change time. We can only manage or organize people, things, activities and events. But time just keeps ticking on its own. I would say budgeting time is a better term. Just sharing what I remember from Steven Covey I guess.
I definitely find the importance of what I'm working on sets my productivity, and when I say importance, I mean to me. Tasks such as beer brewing, where things need to happen on a certain timeline otherwise a bad batch is produced, I tend to be very productive because the end result is very important to me. A task at work however, especially with an arbitrary time line, doesn't rank very highly to me for importance (even if it's important for others), so I tend to be less productive working on the task.