I have three PCs on Lan (all connected by Ethernet cable to a gigabit switch, all run Win10v1903, Build 18362.476, internet speed 100Mbps on all three). When I download the same 5GB file from a server machine A shows 10-12 hours, Machine B 5 hours, Machine C 11 minutes. All machines using their motherboard NIC. A: intel 6700k/Z170 mobo, 64GB 2400Mhz Ram, B: intel 4790k/Z87 mobo, 32GB 2400Mhz Ram, C: AMD 2700x/X470 mobo, 32GB 3200Mhz Ram. Why are Machines A and B so much slower?
first check speed test from each machine one by one
it may be lan cable or network adapter issue
Go to your network and sharing center and check the speed of the your LAN card in the ethernet status compare that with all of other PCs. All the PC should have the same speed It should be 1Gbps. If anyone of the PCs is having 100mpbs or 10mbps. Check the speed & duplex value for your card. Change the Auto Negotiation to a maximum available vault in the dropdown. If you can change the value try with another LAN cable and see the result varies.
If you are just measuring download speed: The complete network stack can influence this, and if you add bufferbloat, it becomes quite complex. Is common when you use a common cable for all.
The cable will likely have no influence (and that’s easy to test by measuring throughput through/from the router , e.g. with
iperf if you have root access on the router).