Pay for your inventory.

[Catwho]

They shouldn't do it more often than once an hour.

Considering the frequency of 90002s, that's unacceptable.

"Hooray! I successfully HQ'd this 6 million gil item with a 5% chance! LUCKY ME!" *crashes one minute later* *stuff is back to raw materials*

Successful completion of the crafted item is determined by RNG on the server, but successful storage of that item on your character is handled by the synch process from your character to your server. 15 seconds is actually fairly slow for a database - most SQL applications measure it in miliseconds. But there are 4 million accounts, of which a million are logged in at any time, which is a million read/writes every 15 seconds.