This is from the 2023 Niro Owner's Manual, which may be different from first generation Niros. There are three different SCC settings shown on page 6-118:

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Even though drivers sometimes cut in on me, I set the following distance to 2 steps below far, as shown in the illustration.
I set Acceleration to 1 step above slow, and reaction speed set to 1 step below Quick. I arrived at these settings by trial and error; others may prefer different settings. Note that the Distance setting is also used if you set Regen braking to AUTO. I'm not sure about Reaction Speed, but this is moot because early on I decided that I did not like AUTO Regen.

 

Yes, it adjusts how quickly the ACC will respond when a) someone cuts into the space ahead of you when you're at an average following distance and b) how quickly it will speed back up when you've been slowed by traffic ahead. I've had cars that were rather abrupt when someone moved in front of me, and unless there's a rapid overtake happening I prefer the car to hesitate a moment before slowing. Many times someone pulls in front of me only to move again into the next lane, so no ACC response is actually required. If I want the car to close the gap up faster than it's programmed, I just press the throttle to move up, then let back off to allow ACC to control it.