A twenty minute video is too long and unwieldy. Break it up into at least three, four sections: and script out what you’re going to say. If that means being silent in places, that’s good; the silent parts are probably going to be the places where you cut the video down further. Good rule of thumb: just because you have to wait for something to load doesn’t mean that the rest of us have to.
Seriously. I just had to skim past almost fifteen minutes of a twenty minute tutorial. I’m not saying that those fifteen minutes were unnecessary to everybody, but the guy could have broken down the original and saved us all some valuable time later.