Could saying "慢慢吃" to a stranger cause offence? I learned that the equivalent to "bon appetit" in Chinese is "慢慢吃" (perhaps also "请慢用"). This is a polite thing to do in English.

But since it literally means "eat slowly", I'm wondering if some Chinese might interpret it literally when coming from a complete stranger. Especially if they are actually eating quickly.

Should I think twice before using this phrase with strangers to attempt to be polite?
Usually you would say "bon appetit" at the beginning of the meal. Same applies in Chinese: you can say either 慢慢吃 / 请慢用 / 请 / 慢用

The only time it might cause confusion is when you say it during the middle of the meal