Maker's Schedule, Manager's Schedule: "For someone on the maker's schedule, having a meeting is like throwing an exception. It doesn't merely cause you to switch from one task to another; it changes the mode in which you work."
This is a good one to remember. It goes back to flow and you can even see it in The Wealth of Nations, while people have seen the existence of the problem for hundreds of years no one has done a good job quantifying it until recently. We have been fooled in school to think that anything can be done in the 50 minutes between bells. They're wrong.
From a cold start it will take 15 minutes or more to get into flow if you are working at it. Most people don't really understand how they do it so it takes even longer. Of course, since most people are interrupted more often then that so they never get there in the first place. If you are in flow and you are interrupted by a short question you can often get back in in a few minutes but a meeting is a total context switch which will drive you back to square 1.
I've been in meetings that were a total waste of time. The worst was a 14 hour meeting that made ZERO (0) decisions. That wasted tens of thousands of dollars and it didn't faze them in the slightest. The first time a manager should do is figure out who needs to be there and how much they cost per hour (all meetings are in hourly increments) and calculate how much the meeting will cost. Then decide if the meeting will generate more value then letting your makers actually make stuff.