In other words, it is more important to mark mines than to clear mines. This is a minor, yet very important difference. You will save a lot of time by concentrating more on flagging mines rather than spending your time clearing other squares.
This is not to say that you shouldn't clear any squares. It would be very difficult if you tried that. However, you should not spend excessive amounts of time clearning squares when there are areas where you can still mark mines.
I mark as many mines in the small area that I am currently working on, then clear a few times very quickly. If nothing opens up immediately, I move on to the next section where I can mark mines. It was this strategy, recently discovered, that allowed me to break the 90-second barrier, and most recently, the 80-second barrier.