In "nasty" terrain, 800 yards is ~45 minutes. Certainly lots of variables, but that's getting in the < .5 mile per hour territory.
When packing out in such conditions, I tend to find it necessary to stop about every 45-60 minutes to rest and readjust heavy packs for comfort purposes.
What I don't understand is the time you are losing to unload, store, hike pack, load your pack back up and retread that same 800 yards. If you're loading your pack up with 70, 80, 100, 130+ lbs, its going to take some time to get that weight all situated. If you're doing this in increments of less than 800 yards, it just doesn't make any sense to me. I'd say that anything less than around 1.5 miles at .5 miles an hour, just knock it out.