[anyfoo]

I'm not sure "slow" matters that much. If (and that's a big if) there is advantage to announce yourself to other civilizations using gravity waves, instead of other means, then it probably does not matter too much if transmitting a reasonable amount of information takes a decade or so. You'd put it on repeat of course, and hope that interested civilizations don't have too short of a lifespan to still make sense of it.