Intel® Nuc has a 2.5 Gbps (i225-V) Ethernet port, but cannot get 2.5 Gbps out of it.
When testing through a demarcation point, the speed reaches up to near 3Gbps.
Resolution
See the recommended troubleshooting below:
Double-check if the established connection speed in the settings can reach 1Gbps. To confirm this, go to Network and Internet Settings. View your Network Properties and scroll down to Link speed (Receive/Transmit).
Connect using an alternate LAN cable, to isolate if the issue is with the cable.
Ensure that Speed & Duplex settings on Advanced tab property of the Intel Ethernet Controller is set to Auto Negotiation.
Check other devices if the same issue is encountered.
If the issue only occurs with the reported NUC, then proceed with the other recommendations.
If the issue occurs across other devices, then contact your Internet Service Provider (ISP) to check if your connection and access point/router. If you bought the access point/router separately then contact the manufacturer of it instead.
Check if you have the latest LAN/Ethernet drivers for your NUC. You can find the latest drivers at Intel Download Center.