Reddit has been very aggressive over the past few years to increase retention. They're barely a link aggregator anymore, they try to self-host as much content as possible. Images, videos, they even attempted to make user-pages à la Facebook (no idea if this caught on). They're competing with Facebook and Instagram, not Hacker News. On paper that's not necessarily bad, but they don't hesitate to make the user experien…
The reddit video player is one of the worst/lagging experiences of any video player since Real Player, and that had bad internet to deal with and Microsoft messing with it constantly. I usually don't like to rag on bad but highlight good, but in reddit's video player they are obviously doing too much tracking in it or it was rushed because it is not responsive many times even on a fast connection on a massive desktop…
Having to watch a pulsating loading icon while it's fetching a few KiB of plain text comments is frankly shameful, especially for a website that's mostly about comments in the first place (I know there weren't comments on Reddit at the very start, but I can't imagine that it would've reached that level of popularity without them).
I think it's a case of poorly aligned incentives, they clearly only want to funnel everybody to the mobile app. The website being an unusable lagfest might be a feature, or at the very least there's probably not a lot of effort put into improving it.