Can you show us the schematic, it would make it much easier to explain to you.
Basically though, there is a capacitor that charges up and then discharges continually. It usually charges and discharges through a resistor: the resistor dictates how fast it can charge and discharge by limiting the current.
Either up the value of the resistor, or decrease the value of the capacitor.
Again, if you post the schematic, it would be very easy to point out to you which capacitor and resistor to change.
There are two factors involved, one is the speed of the oscillator, which is easily changed. The other is the charging rate of the storage capacitor, at fast speeds it might not charge high enough to give full brightness flashes.
However, only one flash per second is exceptionally slow, it sounds like you may have a problem somewhere.