Rate is altitude dependant for most fuel dump systems I’ve known, albeit I haven’t a clue for A380s. The length of time will also have been due to arranging a dumping area which is possibly why the hold pattern goes in two different directions (left hand/right hand) once authorisation had been granted. Given this isn’t an emergency it can take a while to get clearance to dump, both the water and land are rather congested in that neck of the woods and with CBs not too far away and no WX RDR, I doubt they would have wanted to stray too far from a clear patch of sky.