Fast local speed, slow (and unstable) network speed

[Unga_Bunga]

Hello,

First of all, I'm new to anything with NAS or servers in general. If anything is unclear, please let me know.

So, I bought 2 used Fujitsu RX300 S8 Servers on ebay to mess around with and learn a thing or two about Servers.
They have dual 10 Core Intel Xeons and 128GB of DDR3 1600MHz memory.
I connected them via the 10G Port of my 2,5G LAN Switch and my PC to one of the 2.5G Ports.

One of them, I want to use as a NAS (i know there are way better servers for that) and so i installed TrueNAS Scale, configured a pool for 2 2TB HDDs in Stripe mode as i don't really care about redundancy at the moment.

The Problem that I'm having now is that the Read/Write Speed when I'm copying files from one folder to another on the NAS is as expected (it settles at around 200-250MB/s) but when i transfer files over the LAN to my Computer (or Laptop), it varies A LOT. I get anywhere from 2MB/s to sometimes a bit over 200MB/s and i don't really know why. Again, I'm probably just missing something very stupid, as I am a complete beginner

Is there something i can do to maybe test if my network connection is just very unstable, or is it something completely different ? please let me know

If you need addtional information, please let me know

Thanks for your time!

 

[Unga_Bunga]

UPDATE: I figured out that it has to do with NIC that I'm using.

I did some tests with iperf3 and the speeds are looking identical with what i was seeing earlier with the file transfers.
I'm using an intel 540 t2 NIC.

I tested it on the 2nd, identical server with windows 10 and there it is working absolutely fine (constant 2-2.5Gbits).

Any idea why it's working in Windows but barely in TrueNAS ?