"Conventional" flasher relays are designed to flash fast when a lamp is out so that you know something is wrong (failed lamp etc). In effect they sense the total current supplied to the lamps, if it's low then they flash fast. LEDs take much less current than usual lamps so it gives the fast flash rate.
In order to use LEDs with a std flasher relay you need to add enough load for the relay to sense more or less the right current (usually 2x21W load so around 3.5A). Any resistors you add need to be able to cope with this current/power, if you add a resistor at each lamp position they need to cope with around the same power as the std lamp (21W) when on. If you assume 12V then each resistor needs to be (V*2/R) = (12x12/21) = 6.8 Ohms, which happens to be a standard resistor value.
Because they are on only about half the time you can
just about get away with 10W resistors, but 25W will be more robust. Maplins do 10W wirewound power resistors at 48p each
here , they will get hot.
A more elegant solution is to use an aftermarket flasher relay designed for the low loads with LEDs.