Ridiculously slow transfer speeds (2-3MBit/s)

I am new to TrueNAS just over the last few months set up a system and learning as I go. I had hardware issues with the motherboard on the (15 year old) PC I was using, so I got a slightly newer one and the network speeds are so slow I can't do anything useful with it.

General Hardware:
Intel i7 4770 machine
16GB RAM
512GB SSD for OS etc
4x4GB HDD raid-z1

Network Hardware (hardwired):
Network Cable: Cat6
Network Switch: 1GBit
Ethernet controller (per lspci): Realtek...

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