Main advantage why people use iSCSI SANs is that they handle longer transmission distances and are less expensive over Fiber Channel SANs.
iSCSI SAN's performance mainly gets affected by network congestion. Most of the time network congestion is usually the result of an inappropriate network configuration or improper network settings.
For example, a common problem what we see is a switch in the data path into the storage system that is fragmenting frames. It happens most of the time for jumbo frames, and network oversubscription also plays a crucial role there. A slow switch somewhere in the path can reduce the overall speed of the network connection because of this slowest link.