Slow Down, Big Body Ahead
(Wave and Wink)
Jokes aside, "Slow Down" sounds like an order. It doesn't reflect the feedback on why you're slowing down because of a gravity well.
"Slowing Down" would make sense, but wouldn't tell you a reason, and new players would complain because "they got caught by an NPC pirate because they got slowed down by the NPC or for some reason", or people would ask "how can I slow down an NPC like I get slowed down sometimes?".
"Gravity Well" tells you the reason why you're slowing down AND that you're near a body. It's succinct, simple, and gets the player asking "Why does a gravity well/ nearby body slow me down?" and gets them understanding that the FSD behavior is dictated by gravitational effects (effects like Mass Lock and etc).
It's hard to inform important things in as little text as necessary, but that change was a great step towards that AND it also explains the way FSD works, all in two words.