First, was the passenger number estimate correct ? If not, you can deselect the "Estimate passenger number" option in GSX, so you can correct the number GSX calculated from your payload, and insert a more accurate number.
Once you are sure the passenger number is realistic, there's the 2nd part of the problem, and it's fs2crew that starts asking to close doors too soon, since clearly 10 minutes is far too quick to board an airplane, when passengers are moving at a realistic pace and are not simulated.
So, instead of asking how to have the GSX passengers moving quicker than in real life ( like in "hey guys, this pilot uses fs2crew, so we must be quick boarding, move on!!"), the real question would be if it would be possible for fs2crew to allow you more time to board.
Ideally, fs2crew developers might want to have a look at the last pages of the GSX manual, and use the variables we have in place for developers to interface with GSX, so they could simply have fs2crew *waiting* for the last passenger, instead of some amount of time.