HURRY IT UP!
Tell her that dinner is at 7:30 instead of 8. That way you've got a "cushion" of half an hour for her to do all the things she feels she needs to get done, and you're not sitting there steaming about it.
Remind her -- Gently.
An hour and a half before it's time to leave, remind her that it's time to start getting ready.
Get ready first
Do all your showering and shaving long before she locks herself in the can. That way the pressure's off you.
Get a book
The fact is nothing short of tear gas is going to get her out of that bathroom before she's good and ready, so make that time productive. Catch up on your reading. Watch SportsCenter. Whatever it takes to take your mind off the fact that time's-a-wasting.