the difference between fast/quick/rapid

Solution 1:

Macmillan Dictionary defines quick ("able to move fast or do something fast") and fast ("able to move quickly") in terms of each other, so there's not much to choose between them.

Either is a desirable trait in a striking art like karate; force equals mass times acceleration, after all.

On the other hand, Macmillan defines rapid as "happening, moving, or acting quickly," rather than being quick or fast.

Solution 2:

Fast and quick can be used here, and which one you use depends on the facts of the case. An article called The difference between FAST and QUICK in martial arts by ‘Anakonxx’ explains the difference in this context:

Being quick means delivering a technique with explosive speed. Fast is speed without the explosiveness. The former should be the goal of any combat sport devotee.

Some comments in the final paragraph of that article suggest that strength is an important element of quickness:

Lastly, let’s talk about strength in relation to being fast and quick. Once the student learns the mechanics of the technique, the timing is beautiful, your perception speed is marvelous, and your delivery is impeccable. You are only fast, but you are not quick yet. You need to add strength to the strike. ...

If you agree with the martial-arts article’s definitions of fast and quick and the comments about strength, then saying “She is not very powerful” might preclude calling her quick; she might merely be fast.

Some previous answers note that rapid isn't suitable. This is a matter of usage (see ngrams for she is... and he is...) rather than of grammar. I think rapid is commonly used to characterize actions or processes, but not people.

Solution 3:

Quick is best, although fast is ok.

But at least for me, you can’t use rapid here; it doesn’t seem to apply directly to people like that.

Solution 4:

As I understand:

1. Quick: refers to time, it describes something that is done in a short period of time or without any delay;
2. Fast: refers to speed, it describes something moving at high speeds;
3. Rapid: refers to time, but at which rate something changes, that is, either increases or decreases;

That being said:

She is a karate coach. She is not very strong, but she is very quick.

Means that she makes decisions in no time and her movements take short periods of time to be completed.

She is a karate coach. She is not very strong, but she is very fast.

Means that she can run at high speeds.

She is a karate coach. She is not very strong, but she is very rapid.

Is incomplete, and would not get to the original meaning you wanted. It to be changed to something like:

She is a karate coach. She is not very strong, but her students had rapid progress.

Solution 5:

I can't give reference, but just let you know that to my American ear, "quick" or "fast" are both good, but "rapid" seems unnatural in that context.