The word itself isn’t wrong per se, but it has a negative connotation that’s associated with “slow” people (from the medical terminology “mental retardation”). It’s a playground insult to call someone “retarded” because they’re “stupid”. It could just be an American thing, but using it like that is very common.

I think a good comparison is “fag”. Sure, it means a cigarette, but it is (was?) a common insult towards gay people.

Should we not use the word? I don’t know. I’m just explaining why dpeck wrote their comment.

At least here in the US, it's generally no longer considered appropriate.

https://www.specialolympics.org/stories/impact/why-the-r-wor...

Thank you. My wife worked for the Special Olympics for several years, and we were privileged to meet so many wonderful athletes in the program. I doubt a single one would think this usage of the r-word was appropriate.

It's perfectly fine in a technical context: "bacterial growth is retarded by refrigeration" or something of that nature.

But that's not how the commenter used the word. There are other, far less ambiguous and far more polite words to refer to a general lack of progress... old-fashioned, primitive, backwards, antediluvian, obstinate, ...