I don't even understand what people are attempting to critique here? An unreleased console's pre-release OS? We know what it will be like on release. So there was some sluggishness in a video for an unreleased console.
Why wouldn't it run exactly as demonstrated at the reveal on release? People honestly think Microsoft are going to release it while it's running like that? I'm starting to think people found that demonstration more impressive than they are letting on, hence why they've been so aggressive in challenging it.
Sometimes it's worth taking a look at peoples post histories before you question their reasoning, you'll find the same people showing up in a lot of these threads saying the same thing.