Autonegotiation and Duplex Mismatch
Many different modes of operations (10BASE-Tx half duplex, 10BASE-T full duplex, 100BASE-TX half duplex, ...) exist for Ethernet over twisted pair, and most network adapters are capable of different modes of operation. 1000BASE-T requires autonegotiation to be on in order to operate.
When two linked interfaces are set to different duplex modes, the effect of this duplex mismatch is a network that functions much more slowly than its nominal speed. Duplex mismatch may be inadvertently caused when an administrator configures an interface to a fixed mode (e.g. 100 Mbit/s full duplex) and fails to configure the remote interface, leaving it set to autonegotiate. Then, when the autonegotiation process fails, half duplex is assumed by the autonegotiating side of the link.
Read more about this topic: Ethernet Over Twisted Pair