Flasher speed can vary depending on the flasher used. Seems some heavy duty ones can take a bit longer to cycle, probably due to their increased capacity.
Some other factors that may contribute could be bulb current and the general quality of the wiring and connections, as well as the car's available power (11v might cycle slower than 14v).
Make sure your electrical system is working properly then try a few differant flashers and see what happens.
Out of curiousity, why is this important as long as they work?
HD