In case you're curious, I have a guess as to the reason that it takes a few blinks of the lower resistance blinker before it speeds up:
I think it is because the circuit used to do the blinking if probably some kind of Resistor/Capacitor setup, which for the first few blinks can keep the flash rate even (often an R/C circuit can be used to even out minor spikes in voltage or drops in voltage, almost like a buffer), but after a few times the lower resistance bulbs throw the circuit off enough that it speeds up. I imagine the only way to get rid of this "feature"

would be to replace the actual blinker circuit, with one more appropriate for the lower resistance bulb(s).
There, at least you know WHY now, heh. 8)