"Intersect" means the meshes intersect BEFORE the sim starts.

When you run the sim, the clothing should drape/stretch around the figure, and anything else you've set it to collide with (like furniture). But if part of the clothing is inside the figure, even a little, it will be a mess.

If the standing pose is where you started from...that might be the problem. Try starting from the zero pose. You know, the T-pose most figures come in with. The clothing should be designed to fit perfectly in that pose. Your final sitting pose will be the last frame in the animation. So frame 1 will be the T-pose, frame 30 will be the seated pose you want as your end result.

It looks to me like the skirt intersected with the figure's thigh in frame 1, and that's why it took so long and why the results were bad. Starting with the figure zeroed should fix that. Given that the skirt is so tight, you should probably apply any figure morphs in frame 30 as well, at least any that make her hips and thighs larger.