"Isn't it early?"
I think this translates better to "Is it not early?" which is literally asking if it's not early.
"Isn't it early?" in English implies you think it's early but are seeking confirmation or are just drawing attention to the fact.
In Japanese, I believe that you can use a negative question in the same rhetorical fashion you describe for English (this page gives more details: http://maggiesensei.com/2013/02/13/how-to-use-%E3%80%9C%E3%81%98%E3%82%83%E3%81%AA%E3%81%84-janai%E3%80%80%EF%BC%86%E3%80%80%E3%80%9C%E3%82%93%E3%81%98%E3%82%83%E3%81%AA%E3%81%84-njanai/ )
The particle ね (ne) can also be attached to the end of a sentence for a similar effect, as in "はやいですね"
I feel like "isn't it early?' is the wrong translation for はやくないですか. Because if it was early wouldn't that mean it is fast?
This doesn't mean "it's early, don't you agree?"
It quite literally means "Is it not early?" As in "is it late?"
I understand that, but it's not natural in English. Saying "isn't it early?" will make any native english speaker think, "it's early when it wasn't supposed to." So the japanese phrase being translated literally into "Is it not fast?" is contradictory to what an english speaker would think. That's my whole issue with it. If the question was "Is it late?" or "Is it not fast?" (to be literal) would make way more sense.
The problem I think is the japanese here is using as rhetorical question while the English could be interpreted as both rhetorical and simple question.
A similar expression would be いいじゃないですが. In English it would be "Nice, isnt it?" But it is way more easy to tell because it just sounds more natural.
Without the context it would be difficult to tell "Early, isn't it?" as in question or rhetorical. To make the matter worse, you already couldn't tell is it mean "Early" or "Fast".