Hypothetically, if the left Front signal's high filament is out, the current going through the flasher will double since the current only has half the resistance to go through in the same circuit, in this instance, the left Rear signal's working high filament. Essentially, one bulb instead of two bulbs. The higher current will heat up the bi-metallic strip in the flasher faster. Making the bi-metallic strip click back and forth at a faster rate causing the good rear turn signal to blink faster. If you like, switch out the front turnsignal bulbs.
That's exactly opposite of what happens; the resistance of the rear filament doesn't change, and two filaments in parallel are half the resistance of just the one. Half the current through the flasher will cause (in many flashers) rapid flashing, as a bulb-out indication.
If, however, the flasher type we use flashes slower to indicate a bulb out, then something is overloading his, possibly a partial short to ground in the wire to his front flasher.
