The cloth simulation is going to calculate the cloth positiion in steps between frame 0 and frame 30. You can reduce the calculation time by being very careful about what you set as collision targets. for the dress skirt, hip and thigh is about all you need. 

when its done calculating pick whichever frame looks best  and use that for your picture.  Dont apply any poses , you set your final pose at say frame 20 or 25  which leaves 5 -10 frames for the cloth to settle. 

The frame slider is setting which frame is active.

30 frames is often enough for a cloth simulation. If the cloth tears, or is still moving at frame 30, then extend the animation to 40-60 frames, adjust your poses , and try it again.

You can export the cloth at any frame, and import it as a prop. It will retain whatever shape it is at that frame.