Don’t use dd to test read/write speeds, use fio - google it up
Make sure you run fio from a dataset on your zfs pool
You to check/do these things :
Check MTU size and set to 9000 on NAS, 9014 on Windows
Check whether SMB multichannel is enabled on both TrueNAS and Windows sides.
Set-SmbClientConfiguration -EnableMultiChannel $true
Check whether RSS is enabled on Windows
Get-NetAdapterRSS
And check that you see multiple connections on Windows to TrueNAS’s share :
Get-SmbConnection
Get-SmbMultichannelConnection