Replaced factory cluster w aftermarket. Turn signal issues

I recommend focusing on what has been changed. If it worked before, the chances of it failing as you changed the instruments is highly unlikely.

As far as the flasher goes, it wouldn't blink slowly due to the LEDs. If a bulb burns out, it flashes really fast. The burned out bulb has reduced the load on the circuit and now the flasher hits it's amperage faster. This causes the thermal contacts in the flasher to open much quicker. LEDs reduce the load quite like a burned out bulb would. The LED flasher has a higher amperage to trip which slows the flash rate.