They do slow down the metabolism, yes.

They seem to also act on some reward mechanism - making some stuff (not just food. Alcohol, gambling, porn, smoking, other drugs have anecdotally been reported to feel less attractive / addictive) have a reduced pull on one's will. They also measurably delay gastric emptying - to the point that some medicines, meant to be taken on an empty stomach, are not safe to take within up to 3 hours from the last meal, compared to one hour or so for other people.

It’s not clear to me that they slow down metabolism consistently, although that does seem to be the effect in some people.

I rather think the opposite is true, that by increasing signals that the body is satiated in many cases it is avoiding a metabolism slowdown caused by hunger.