A similar question has been asked.
However, is it possible to give (general) differences in usage of fast, quickly, speedy and rapidly?
And with respect to the top answer: Are quick and fast absolutely interchangeable?
I do not think it is strictly possible to give guidelines to usage that are anything other than pedantic. I do, however, think that etymology provides some helpful clues to the more natural contexts of usage.
Quick is related to life and living things such that "quick as a rabbit" seems natural.
Fast has its relationship to strength and force that makes "fast cars" and "run a fast race" seem like a good fit.
Speed is related to success and goals giving "speed limit" and "speedy delivery" their purposeful ring.
Rapid has a hunger to it that makes it well suited to phrases like "consume at a rapid rate" or "rapidly overtaking."
Mixing these up produces some odd results: "Rapid rabbits" are comprehensible but more predatory that we are used to rabbits being. "Quickness limits" Nonsense. "Quick delivery?" Something else entirely. A baby has been born. Further evidence that "quick" wants to talk about life.
There are certainly no rules, just clues buried in the historical antecedents of the modern words. I would say try "quick" first for natural systems and living things and use "fast" as the go-to for machines, forces and acts of power. Rapid has a dark side. Speed is ambitious.
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