Both synonyms are correct. You can say "Quick!" or "Quickly!" to mean "hurry up." "Quick!" is a more neutral way of saying it, and more commonly used. This is because "quickly!" is usually said when you are annoyed at someone so it may come across as insulting. "Hurry up!" also has the same demanding tone as "Quickly!" so many English speakers only use "Quick!" unless they are annoyed. "Abbreviating some words: it's faster to read." This is perfectly right.The only other substitute for this word would be "quicker." When you add 'er' to the end of these words, it means 'MORE fast" or "MORE quick." Since 'rapid' is not in this form, and cannot be put into this form, because there is no such word as 'rapider', you cannot substitute this here. The only way to use rapid in this sentence would be to literally say "more rapid" which sounds very unnatural. So people only use 'faster' or 'quicker' generally when they want to compare speed. Rapid is commonly used when describing the sea or other large objects. It can be used for describing small objects too but the word 'rapid' has a meaning of great force, or powerful speed so that is why it is used when describing big objects, to give that effect. And it is mainly only used for describing that something is moving fast, rather than for comparing or to say it is going 'faster' than something else. This is done through it's 'adverb' form, 'rapidly.' For example "The waves of the water crashed rapidly." Or "The fan spun around rapidly, cooling down the room." But when you want to compare speed, you would use 'quicker' or 'faster' For example, "The waves of the water crashed faster than anything she felt before." or "The fan spun around quicker than her older model, making the room much cooler." So in summary: For simply stating speed: Quick/Quickly Fast Rapid/Rapidly For comparing speed: Quicker Faster I hope this makes sense.