According to Reddit's "Staff Platform Engineer (Web Platform Team)":

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Old Reddit has the advantage of being pretty much static non-interactive content. No video, tiny thumbnails, and barely any JS or styling. Some people like this and some don't, but the end result is a very lean website that performs well out of the box.

https://x.com/jimsimon_/status/1841087335414280571

Suffice to say, I'm on the frontend perf team and we're acutely aware of these problems

https://x.com/jimsimon_/status/1841092341991403974

--- end quote ---

This was in October 2024.

Which is of course a bunch of bullshit when you consider that Reddit's backend returns most data in under 400ms, and it takes Reddit frontend 3+ seconds to render it

It could be that they are just incompetent.

Based on how universally horrible their video playback performance is (even in the iOS app), I can only assume incompetence.