Most casual viewers of anime and learners of Japanese are familiar with the word sugoi (すごい), roughly equivalent to “tremendous”. However, as a matter of grammar, it’s not good to write “sugoi hayai”. That’s hayai (速い) for the adjective “fast”. The reason’s simple: “sugoi” is an adjective too. It doesn’t mesh.
When using the root of “sugoi” to add detail to “hayai”, the spelling changes to fit this new role. That’s why we use sugoku (すごく、凄く) to fit the bill. My post title’s “incredibly fast” illustrates the grammar: “incredibly” modifies “fast”.
So what if we change the grammar and change “fast”, an adjective, into “speed”, a noun? Well, in English, the “ly” is dropped and we get “incredible” back as a pure adjective.
Japanese isn’t much different. Speed itself would be hayasa (はやさ、速さ). The ending sa (さ) makes nouns out of various Japanese concepts, with speed being only one. Another example would be omosa (おもさ、重さ), for “weight”. (And no, I do not mean “mass”, Japanese has another word for it. Mass is not weight, as we know from high school science.)
So our result is:
Sugoi hayasa (凄い速さ、すごいはやさ）。
Where would you use the first, then? Well “hayai” being an adjective by itself, it must be modifying something. For example:
(Suupaaman ga sugoku hayai = Superman is incredibly fast!)
Faster than a speeding bullet, perhaps? – J