The issue is slow gastric emptying combined with generally reduced gut motility.
Last year I had 2 of them. The second came about because the first prep was bad — and sadly the second was bad as well. By "bad prep" I just mean poo still sitting in the colon, which makes it hard to see everywhere. On the first I stopped the GLP; on the second I didn't. Even extra enhanced prep on the second still wasn't enough. A third time was never going to happen in the end, since they saw nearly everything and took 16 biopsies over the 2 scopes — enough to assess the ulcerative colitis and confirm there were no cancers. Small patches of dysplastic pre cancerous areas could have been missed, a slight risk, but not a high one.
So let them know. Enhanced prep may be what they want to use, and they may or may not ask you to stop them a few weeks ahead. Any constipation you have is worth pre treating from a week before. Fluids only a full day early is something else I'd consider, along with kicking the prep off a few hours sooner. That's what I did, and retained contents were still in my stomach when a gastroscopy was done at the same time — not ideal, given the aspiration risk with the sedation. Getting the 4 liters of prep solution down was very slow for me too; 7 hours was how long it took, simply because the slow gastric emptying meant I couldn't drink more — another reason to maybe begin a bit earlier.