Well, if you receive the url using gmail, google will visit the link, and execute the JS code. If you don't have a grace period, your secret will be unreadable before the human as a chance to get to it.
Just saying that because, while at 0bin we have a 1 day default expiration delay, users can chose burn after reading. But we had to add a small grace period to allow for user to check the URL, reload by listake, send vua gmail, etc.
I thin ots burn after reading by default so it could affect you.