The user is potentially switching back and forth between apps (think copy+paste to create cards). If the one month period elapses at an inconvenient time, the user may end up having to wait for the optimization to finish, or will need to repeatedly cancel it each time they switch back.
That benchmark ignores the I/O costs of reading that data from disk. If the user’s disk isn’t able to respond to that request promptly, answering the card is going to block until it completes.