Full comprehension just means you gathered the intent of the point the writer was trying to get across. You can easily do that with 20% of the words they wrote if you pick the right words. Scanning is basically training to find all the important words to fully piece together the idea as quickly as possible.
So 25 words per second may sound like alot, but it's really the time necessary to find 5 words per second out of the crowd of 25. And like any other skill, it is something you get significantly better at with practice. Lawyers are gonna have alot of practice if they work on a skill like this.