Oh dear God.  This mission absolutely destroys my machine.  In game, immediately after start I'm getting something in the region of 1 frame per second ... but when I try to go back to the menu (my plan at first was to try dropping some graphics settings) it takes 15 seconds to redraw the menu screen, and at least 5 seconds to move the mouse cursor one step.  In the end it was all I could do to back out of the game in an orderly way: move the mouse, wait for new position to draw, move, wait, rins