From what I read, the 1 to 2 lb guide is not meant for morbidly obese people but it gives us a clue. The reason is that your body won't let go of more than 1 to 2 lbs of fat in a week for someone who is overweight/obese. The rest of the weight will come from muscle which is what will lower your metabolism and is usually what is blamed when it comes to yo-yo dieting. Ie you lose fat, you lose muscle, you go back to eating "normal", your metabolism is lower than before so you gain all the fat back plus more.
Now for someone who is morbidly obese, fat loss can come faster because there is just more fat to lose. The key though is to try to minimize muscle loss as well. I don't know if there is a true formula based on fat percentage for ideal amount of fat loss per week, but if there was that'd be a good way to figure it out. During my initial weight loss, I steadily lost 65 lbs over a 3 month period which is about an average of 5 lbs/week. I was weight training, increasing strength and eating around 2000 calories/day. From my body fat percentages taken before my weight loss and afterwards, it seems like I didn't lose much muscle at all in that time. Now that I don't exercise as much but I still do a mix of weight training and cardio and still eat around 1800 calories/day, my average loss is about 2 lbs/week.
I figure my weight loss has slowed down because I have less fat to lose and I'm not as gung ho on the constant exercising as I once was. I'll happily take 2 lbs/week or even 1 lb/week