The turn signal circuits are current “detecting.” If a an incandescent bulb blows out, the corresponding drop in current draw is what triggers the hyper-flash you are seeing. LED bulbs draw far less current and are “seen” as a blown bulb. The traditional fix is to install a high-current resistor in parallel across the electrical connections to the LED bulb. The resistor draws current similar to the incandescent bulb and tricks the system into not hyperflashing the LED. There are two drawbacks : the added resistor prevents any efficiency gains that he LEDs would have introduced and, once installed, your turn signal system will never go into hyperflash, even if the LED bulbs fail. This means you will need to be extra-vigilant in checking your LED bulbs to make sure they are working. I check all of my LED bulbs monthly.
Alternatively, there are replacement turn signal relays that don’t need the resistor in parallel with the LED and still flash at the normal, slower rate. I’ve been interested in these, but time and lack of grey matter have prevented me from digging any deeper applying this technology to my Smart.