The time comparison only comes into effect if the key sent is wrong.  If someone is spamming topic in order to get the key they can send it as fast as the server can respond.  If the server starts throttling responses it's indistinguishable from the server lagging, it makes it a step harder to create a bot to brute force your key.  While there are ways around this it is just one more layer of protection stopping script kiddies.