VMs are slow accessing an iSCSI device. You've checked the host, network, VM, and physical storage and nothing's wrong. What might be the problem?
The ESXi host might generate too many commands to a LUN, causing some to be queued by the VMkernal, increasing latency. Usually a misconfigured setting of LUN queue dpeth is the cause. Determine the proper LUN queue depth from your storage vendor documentation and then adjust Disk.SchedNumReqOutstanding accorrdingly.