During startup, vSphere Replication hbrsrv needs to try to connect to all hosts in the vCenter inventory. If customer has a large environment and a single hosts can not connect to hbrsrv, it will take long time to loop through them. Network firewall may add unnecessary time to HBR service startup.
If an ESXi host is removed from the VC inventory while the vSphere Replication hbrsrv is offline that will also interfere with the startup of the service as vSphere Replication hbrsrv still has an ESXi entry for that host in its database.