A reply sent instantly can leave the real question unanswered just as quickly.

Speed gets mistaken for responsiveness. A fast answer clears the message from your screen and returns something to the sender, but if it addresses the easy part and skips the difficult one, the thread simply continues with the same problem in it. The other person now has confirmation that you read it, and still no help.

Before replying quickly, find the actual question and check whether your answer touches it. If it does not, say so and give a time when it will. A slower reply that resolves something is friendlier than a fast one that does not.