This is exactly correct. The standard method is to use a 6ohm 50watt resistor per bulb, and there are plenty of these available at different stores for this very reasonOn newer cars, some folks that replace their incandescent signal lights with LED bulbs have the same fast flashing of the signal light. I suspect this is because the LED bulb uses a fraction of the current of an incandescent bulb, so the computer thinks the bulb is burned out, and goes into fast flash mode.
The solution for this issue is the installation of a resistor in series with the LED bulb, to draw a larger current load, to simulate the stock incandescent bulb.