Ethernet LAN speed too slow on one machine
This problem has the feel of a data packet size mismatch causing fragmentation with corresponding delay. Possibly combined with the machine hardware for some reason being set up for half-duplex communication. 
For more information about packet size you can use the ping command from terminal with different packet sizes as described in this article: https://community.hpe.com/t5/Networking/What-detail-symptoms-will-I-be-getting-if-MTU-size-mismatch/td-p/6909407
Compare the ping results between the fast and the slow computers on the same physical connection.