You could decrease the resistor coming out of pin 6 of the 2399. The problem, however, is that you'll run the risk of the chip stalling, for lack of a better term, when you connect the circuit to power. It simply won't work when this happens. The fix is to disconnect then reconnect power and should be good to go. Thus is why nearly all 2399 based delays have a 1k resistor or higher in addition to the TIME or DELAY pots.