Ask HN: What tech choices make “new” Reddit so unbearably slow?

I never visited Reddit much, but occasionally web searches take me there for tech support questions. Every time I visit in the past year or so I'm shocked at how slow the interface is, and don't even understand how its possible to make web sites so unresponsive to user input. It's as bad as visiting a local news site that is laden with ads and several auto-playing videos, but it doesn't seem to be running that much ad content.

How does one make such a bad web app, and what technology choices should I avoid to make sure I don't end up with a mess like this?