More on the super-slow Ethernet problem, which it sounds like @pathusa92 is also experiencing ...
I booted my NUC8i7BEH from a Live Ubuntu 18.04 USB and tested copying some files.
Ubuntu has the same problem!!!
(update: I tested Ubuntu at work the next day. Still bad. Ping latencies are 10x-100x slower than the Win10 machine on the same switch)
When I plug my Ethernet cable / switch into a different Windows 10 computer, no problem copying files over the network, so it seems most likely that the problem is with the NUC8's Ethernet hardware or the driver.
The Mojave 10.14.5 NUC8i7BE with IntelMausiEthernet.kext v2.4.1d1,
or the Ubuntu Live 18.04 can both ping the network and do some minimal web browser stuff, but if I try to copy ~ 200MB from an SMB share, the copy seizes and fails. Watching ping latency times go up in the 1000ms range.
I'm worried I have some bad Ethernet hardware in my NUC8!! Though an alternative thought is that the IntelMausiEthernet.kext and Ubuntu Ethernet drivers have the same bug, which seems plausible, as I read
here, that the IntelMausiEthernet.kext is based on Linux source code.
https://www.insanelymac.com/forum/topic/304235-intelmausiethernetkext-for-intel-onboard-lan/
Ubuntu Live 18.04, as best I can tell, from looking at the dmesg output, is using the Network driver
e1000e: Intel(R) PRO/1000 Network Driver - 3.2.6-k
I read
here that there is a version 2.5.0d0 of the IntelMausiEthernet.kext,
but i have not been able to find a download for it.
https://github.com/Mieze/IntelMausiEthernet/commit/f3c69cec20efd24fa467cf16f44ccaae61336766
(Update: I obtained & tested the v2.5.0d0 IntelMausieEthernet.kext, but it did not fix the problem)
I will have to install Windows and test the networking, to rule out bad hardware
Fortunately, I backed up the SSD before erasing it, to install macOS. I never bothered setting up the OEM Windows 10 that came with the machine. This is all very slow going as I only have time to work on this intermittently. I need to test this ASAP, in case I have some bad hardware. I've had the unit a couple weeks and worry I may need to return it as defective if Windows also has a problem.
======================
Update: A couple days later.
Strange development! I received my TRENDnet TU3-ETG USB Ethernet dongle. I installed their 10/19/2017 Mac Driver for macOS 10.9-10.13 (even though this is 10.14.5) from
www.trendnet.com
and reboot. Plugging the Ethernet cable to the dongle, the network works well, and ping latencies to 8.8.8.8 are in the 5-7ms range. I can download Clover very fast. I then plug the Ethernet cable back into the built-in Ethernet jack and wait a few seconds, the network reconnects. I test downloading Clover a couple times, and this too is fast, though the ping latencies to 8.8.8.8 are fluctuating wildly in the 100-1000ms range. I then unplug the dongle and the Ethernet performance goes back to the unusable state. PRETTY WEIRD!!
Anyway! I need to give this quest a rest for a while and move forward with just using the Ethernet dongle that seems to download ok and shows reasonable ping latencies. I hope to revisit this mystery later.