Flasher units work on resistance, so if it's going fast it is seeing a high resistance when switched on to that left side. As no bulbs have gone, then as mentioned above – have you fitted LED replacement bulbs? If so, then you will need a ballast resistor wired in parallel to the bulbs to simulate the expected load. If you have normal bulbs, and they are flashing OK, but faster on the left, then you undoubtedly have an earth issue. One of your left-hand indicator bulbs is not earthed directly. Most commonly happens with tail lights. Check all the wiring from the left-hand rear cluster, and make sure it has a good earth.
What type / age of car is it?
Do you have a towbar fitted (and wired)?