We got an new 1 Gbps fiber internet this week, after which we found out that the black Belkin USB-C to Ethernet Gigabit adapter attached to the hub on the backside of LG UltraFine 5K gave only 200 Mbps download and 500 Mbps upload throughput (latency was good though, 3-4 ms). Changing ports didn't help, couldn't test the adapter straigth to MacBook Pro 13" because 2015 model and no USB-C ports. With other Thunderbird to Ethernet Gigabit adapter the throughput was good 940 Mbps / 940 Mbps.
We updated the firmware of the UltraFine 5K to the newest, no difference in the throughput, update took around 15 minutes, quite long.
Then we updated the driver for macOS, black Belkin F2CU040 has Realtek chip inside, so we went with this:
MAC OS 10.6 to 10.13, version 1.0.17 2017/12/19 184k
http://www.realtek.com.tw/Downloads...=56&Level=5&Conn=4&DownTypeID=3&GetDown=false
After the driver update (and restart of the macOS), the throughput is 940 Mbps / 940 Mbps.
The UltraFine 5K is an excellent display, this was the first problem in the last year of using it, no other problems or bugs that we have found out.