Here are some disk performance measurements (made on a Windows 10 VM on vSphere 6.5 with CrystalDiskMark) across different types of datastore (local SSD array, NFS (sync), NFS (async) and iSCSI).
The NFS and iSCSI datastores were on a Synology RS3412xs with 7x 2TB Samsung 860 EVO SSDs in a RAID5 volume. The vSphere host was connected to the Synology with 10Gbe.
The vSphere host was a Dell 720 with a PERC H710 mini disk controller (with battery-backed cache) and 4x 2TB Micron 5100 SSDs in RAID5.
Note: Test 4, using asynchronous NFS, is not relevant for real-world VMs because a power failure is likely to corrupt VM file systems on asynchronous NFS volumes.
Test 1: Local SSD array datastore
Test 2: NFS (synchronous) datastore
Test 3: iSCSI datastore
Test 4: NFS (asynchronous) datastore
Note! only included for comparison – async NFS is not suitable for production VMs (since a power failure will corrupt the VM’s file system due to the fact that file systems journaled writes may not be flushed to disk).