The difference in blinker speed maybe the flasher.
Put simply, the flasher is a type of circuit breaker that opens and closes when an electrical load is applied to it. The greater the load the faster it opens and closes. When signaling for a turn, the load applied is half that of the 4 way function, so the flasher opens and closes at a slower rate while signaling for a turn.
Way back in the last century, most cars had 2 flashers, one for the 4 way operation and one for the turn operation. Each flasher was calibrated so that the 4 way flashers blinked at the same rate as did the turn signals.
I'm not sure if the 2 seperate flasher system still holds true today, but if the system only has one flasher, it would account for the difference in time it takes the turn signal to flash when compared to the time it takes the 4way to flash.
However, if the system has a separate flasher for the turn and 4 way, the difference in flash time is due to the calibration of the flashers being the same. If both flashers have the same calibration, one flasher will open and close faster when the 4 ways are in use.
The result of having 2 flashers with the same calibration will be the same as having only one flasher for both operations.
