Check the ground and the bulb for the left rear. More specifically the ground. If it looks crappy, clean it off with some scotch brite, or a wire brush and reattach it and see if it helps. A corroded ground would be causing high resistance in the circuit. High resistance equates to low amperage passing through the bulb, it also could cause the bi-metallic strip in the flasher unit to not heat up as quickly as it should, thus the slow blinking. Try swapping the bulb on the right front with the left front bulb or a new bulb. The symptoms for that one directly resemble a burnt bulb. A broken bulb filament would cause less resistance in the circuit, less resistance equates to higher amperage. The higher amperage heats the flasher units bi-metallic strip quicker than normal, in turn causing your increased rate of flash on the right side.
Hope this helps, FWM.