Why does mailgun take so long to check an updated DNS?

So, I understand that DNS propagation takes time. But isn't it possible to just clear the DNS cache wherever it is?

And another thing is, suppose I add a new DNS record, now, if it is an updation, then the change needs to be propogated around the world and so on. But if it's a creation, no one really has the record yet, so, they have to come to the root server to get everything anyway. So, why does addition also take some time? (I see that this time is less than the updation time, but it still takes some time)