Turn signal flashers are depedant on current goin through them, the more current the faster they flash. the HD flashers you are putting in are probably the normal flashers with a different cover. See if you can rig a hazard light flasher in or try a different brand of flasher. A true HD flasher is the same thing as a hazard flasher. Hazard flashers are more like relays, one switch turning a different bigger switch on, that way they are non dependant on current.