There's deceleration involved as well. Here's a very nice video of the shuttle boosters on their way down to give you an idea of the velocities involved. https://www.youtube.com/watch?v=2aCOyOvOw5c
An enormous amount of inertia to overcome so 30% doesn't sound so unlikely.
Indeed. Combined, these all burn quite a large volume, but #1 is the most costly in terms of fuel, I think. It's essentially a reversal of the flight path.
Not quite reversal since it doesn't negate the vertical motion, but yeah. Definitely the most significant of the three burns.
Though it's not quite that simple. Based on the trajectory of last week's launch, they're actually going to use a much steeper trajectory during the first stage launch. The result is that the first stage uses significantly less propellant to make that first burn (since its downrange velocity is much smaller), but it cuts into the payload capacity quite a bit by forcing the second stage to do more work.
An enormous amount of inertia to overcome so 30% doesn't sound so unlikely.