It's absolutely substantiated. I can easily reproduce the phenomenon myself using curl from the command line. The key is to use a web browser User-Agent (curl -A), because it doesn't happen for curl's own User-Agent. The delay is consistent and only affects certain Musk-hated domains such as the NYT and Threads. Here's an example, try it yourself:

  curl -v -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.6 Safari/605.1.15' https://t.co/d8KWBqT5v6

How do you explain the delay being trivially reproducible for, say, NYT and consistently absent for Fox News?

It was substantiated by a few dozen HN users, and the Post journalists themselves: X is throttling traffic to some sites and not to others.

It appears this throttling is based in part on the useragent since at least one HN comment noted that the throttling was observed when using curl spoofing a mozilla user agent.

I don't know why you are being downvoted. It is completely unsubstantiated nonsense. Just because some sites seemingly have had a 5 second delay when opening doesn't prove anything. Yes, some of the sites it was happening to Elon might not like. But you have no idea how many sites it was happening on. It could have been affecting more sites Elon likes than he dislikes. But no one can prove otherwise, so it is by definition unsubstantiated.

Even using the word throttle is not an overly apt description. You can still open as many links as you like, there was just some delay. (semantics I know)

Down vote me too, but there is zero proof this was nefariously enacted.

As techies, we should know systems fail often in bizarre ways.