It is assumed that the rate of the IGMP/MLD state change on the Vport level is substantially lower than on the subscriber level.
The reason for this is that the IGMP/MLD Join/Leaves are shared amongst subscribers on the same Vport (PON for example) and therefore, the IGMP/MLD state on the Vport level is changed only for the first IGMP/MLD Join per channel and the last IGMP/MLD leave per channel.