Most of us at a certain point will have at least weakened pelvic floor muscles. Prolapsed pelvic girdle region organs can cause a myriad of symptoms. It's the degree to which they are prolapsed that causes the problems or lack of them.
If your prolapsed organs include the bowel and rectum (as many do), it can create a "pouching" of the lower bowel/rectum which retains fecal matter and can cause constipation or a slower emptying of the bowel. If the bowels are emptying more slowly, it can slow down the entire digestive tract system and cause slower gastric emptying. Is this what is happening in your case? I don't know.
More than likely, there is also something else causing the stomach not to empty as quickly as it should. It could be a nerve problem or, it could be a systemic situation that causes a slowed digestive system.
Medications such as Propulsid can speed up gastric emptying. You didn't mention the reason for your Upper GI but, my guess would be that you're being tested due to some reflux problems. Am I right on that? Many people nowadays suffer from this type of slowed gastric emptying due to our diets and lack of fibre, too much refined/processed foods and, or too much fat content. Our systems become sluggish because in all reality, these types of foods are rather "toxic" and our bodies don't handle them well.
My suggestion would be to talk to your doc about this more thoroughly. You may not even realize that you're somewhat chronically constipated because you're having bowel movements. The question is, are you truly emptying your bowels fully?
Try upping your diet levels of fibre, especially whole grains, vegetables, fruits etc.. You may even talk to your doc about adding a fibre supplement such as Metamucil (they also come in pill form now so, no messy drinks to down :)) And, try to get as much exercise as you can. Even a daily 20 to 30 minute walk if possible does wonders in getting our digestive tracts back on track. Also, try some Probiotics by going to your healthfood store. They also come in capsule form and help restore healthy bacteria to get your intestines working again. The combo of fibre and probiotics are GREAT at keeping things balanced in our digestive tracts.
Short answer is that it's likely not JUST the prolapsed organs, if at all, which is causing this. There are all kinds of other causes so, best talk it over with your doc as there are remedies to this both natural and medications.
Keep us updated and let us know what you find out as it will help others with the same problems. :)