rapid
When used as an adjective, "rapid" often describes a process or change that happens quickly (like "rapid growth"), whereas "fast" is more commonly used for physical speed (like a "fast car"). When used as a noun, it almost always appears in the plural form ("rapids") to describe the turbulent sections of a river.
💬Casual Conversation
🎬Tuesday afternoon, Fatima is in the library while Maya is at a cafe.
Maya
The price of these lattes is seeing some rapid inflation. I'm over it.
Fatima
Just cut back. I'm totally swamped with finals and can't even think about coffee.
💡
Maya uses 'rapid' to describe the quick increase in prices, while Fatima uses the idiom 'swamped' to indicate she is overwhelmed with work.