Interesting. Its reacting with the voltage. I can see one rate at 12.6ish battery voltage, then maybe a slight change at the 14.2ish running voltage, but it shouldn't climb after that unless your regulator isn't working like it should.

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I seem to remember reading about a need to add diode(s) to the dash turn signal circuitry to obtain normal flashing.

edit: VN900 DIODE MOD

 

The diode eliminates the excess current from bleeding through to the other side. It does nothing to regulate the flash frequency. Something in the flasher relay or voltage input is goofy. The OP needs to take a quich measure of voltage at the battery when running and at various RPMs and eliminate that possible option.

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I have done this mod to my 900. If I remember correctly, the 2 diode mod isolates the left side from the right side, which isn't necessary in the incandescent config. So the speeding up and slowing down of the flash rate (voltage sensitive) could be caused by the load, flashing both sides. If you already have both sides isolated with the diode mod, then I would say replace the flasher with a different brand made for LEDs. There are lots out there that will work. Something could be wrong your current flasher. They are also fairly inexpensive.