Read only LUNs are typically used for replication purpose and are commonly seen in vCenter Site Recovery Manager (SRM) environments. In a production environment, ESX/ESXi hosts heartbeat all available VMFS volumes, which fails on the read only VMFS LUNs.
In the Add storage wizard, the ESX/ESXi hosts query all available storage to see the devices that are available. If there are read only LUNs that are in a read only state, the scan for available devices take a longer time than expected.