In some cases, depending on infrastructure setup and iSCSI storage, you can experience an issue when HOTADD mode performs slower than NBD(network) mode.
Some iSCSI storage arrays from different array vendors do not behave appropriately during periods of network congestion. This problem is related to the TCP/IP implementation of these arrays and can severely impact the read performance of storage attached to the ESXi/ESX software through the iSCSI initiator. The problem has also been reported in native (non-virtualized) environments involving the Microsoft iSCSI initiator.
Please follow VMware KB and disable TCP delayed acknowledge