The rate the indicators flash at is governed by the wattage of the bulbs in the circuit. The LEDs you have fitted will have a lower wattage than the bulbs you replaced them with hence the faster flashing.

When you fit LEDs all round you need to replace the flasher relay with a different one but as you have only replaced the rear indicators you probably need to solder a resistor in series with the rear indicators to effectively increase the power needed to make them work and that will slow the flash down again. As to what rating of resistor you need I don't know I am afraid but the above is deffo your problem.

Cheers

Martyn