Are you using the right tool for the job? If I wanted to remove a lot of material in a hurry, I'd use my 3x21" belt sander. I have belts from 24-grit up to 320-grit. With the 24-grit belt installed and running at 1100sfpm, it will remove a whole lot of material in a hurry!
If you're using a random orbit sander or a finish sander, you might look for 36-grit sandpaper. You might also try switching to some sort of scraper. Perhaps a heavy-duty paint scraper?