I know I have many kinds of inefficiency in my working methods. Since I live in two places, I often don't have access to the book I need when I need it. There are distractions, etc..., so that time is not always used efficiently. It would be depressing to think that I am working at 100%: that would mean that I've already done everything possible to reduce any wastage of time. I like the feeling that I'm maybe at about 40% of the maximum. That means that if I really wanted to, I could easily increase my productivity any time I wanted to. If I really need to do something in hurry, I can do it, because there is extra slack there. Also, I can take my time and exploit the fact that even wasted time spent on the project is not really wasted.
Once in while I look at some things I'm doing and try to make them more efficient, but I'm not at all obsessive about this, because I like the feeling of some degree of extra capability that I don't always have to use. Recently, for example, I unsubscribed from some email lists I was on and organized my files on my computer desktop in a more rigorous way.
So the SMT of the day: be efficient, ma non troppo. If you are already getting done what you want, you can increase efficiency in order to get the same amount of work done in less time, but you don't necessarily have to aim at 100%.